Re: [GENERAL] SSDs - SandForce or not?
On 11/14/12 2:11 AM, Toby Corkindale wrote: So on the face of it, I think the Sandforce-based drives are probably a winner here, so I should look at the Intel 520s for evaluation, and whatever the enterprise equivalent are for production. As far as I know the 520 series drives fail the requirements outlined at http://wiki.postgresql.org/wiki/Reliable_Writes and you can expect occasional data corruption after a crash when using them. As such, any performance results you get back are fake. You can't trust the same results will come back from their drives that do handle writes correctly. I'm not aware of any SSD with one of these compressing Sandforce controller that's on the market right now that does this correctly; they're all broken for database use. The quick rule of thumb is that if the manufacturer doesn't brag about the capacitors on the drive, it doesn't have any and isn't reliable for PostgreSQL. The safe Intel SSD models state very clearly in the specifications how they write data in case of a crash. The data sheet for the 320 series drives for example says "To reduce potential data loss, the Intel® SSD 320 Series also detects and protects from unexpected system power loss by saving all cached data in the process of being written before shutting down". The other model I've deployed and know is safe are the 710 series models, which are the same basic drive but with different quality flash and tuning for longevity. See http://blog.2ndquadrant.com/intel_ssds_lifetime_and_the_32/ for details. The 710 series drives are quite a bit more expensive than Intel's other models. Intel's recently released DC S3700 drives also look to have the right battery backup system to be reliable for PostgreSQL. Those are expected to be significantly cheaper than the 710 models, while having the same reliability characteristics. I haven't been able to get one yet though, so I don't really know for sure how well they perform. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Unexpectedly high disk space usage
On 11/7/12 3:58 PM, Jeff Janes wrote:
WHERE nspname NOT IN ('pg_catalog', 'information_schema')
I question the wisdom of that where clause (from the wiki)
If the pg_catalog relations are big, then they are big and why
shouldn't they get reported as such? And if they are not big, that is
why the limit is there on the wiki page.
The idea was that in a new database with a relatively small number of
tables, your own tables will be lost among the catalog data unless you
filter them out. Testing against an install with a single real table,
the query there will show something like this right now:
relation| total_size
+
public.t | 3568 kB
public.t_k_seq | 8192 bytes
But if the filter on pg_catalog is removed, you get this instead:
relation | total_size
-+
public.t| 3568 kB
pg_catalog.pg_depend| 808 kB
pg_catalog.pg_proc | 752 kB
pg_catalog.pg_attribute | 568 kB
pg_catalog.pg_rewrite | 464 kB
pg_catalog.pg_description | 392 kB
pg_catalog.pg_statistic | 328 kB
pg_catalog.pg_operator | 208 kB
pg_catalog.pg_collation | 152 kB
pg_catalog.pg_type | 152 kB
pg_catalog.pg_amop | 136 kB
pg_catalog.pg_class | 136 kB
pg_catalog.pg_constraint| 112 kB
pg_catalog.pg_conversion| 104 kB
pg_catalog.pg_index | 88 kB
pg_catalog.pg_amproc| 80 kB
pg_catalog.pg_opclass | 80 kB
pg_catalog.pg_ts_config_map | 80 kB
pg_catalog.pg_cast | 80 kB
pg_catalog.pg_authid| 72 kB
That is overload for a lot of people, and confusing to new users.
That's why I opted for the shorter version.
There's no perfect answer to all use cases here. This sort of thing is
why there's three sets of queries for pg_stat_user_tables,
pg_stat_sys_tables, and pg_stat_all_tables. The wiki disk space queries
aim to be like the user tables version from that trio.
Adding a note pointing out that you might want to remove pg_catalog and
see the size of those relations would be appropriate. I wouldn't make
that the default case though, due to the issue highlighted above. I'd
rather optimize the initially suggested query so that new users get
simple output, even if it means that might hide problems on larger
installs, where the catalog data became big.
The other way I sometimes balance these two requirements--want to show
all the big data, but not clutter small installs with the catalog--is to
make the filter size-based instead:
SELECT nspname || '.' || relname AS "relation",
pg_size_pretty(pg_total_relation_size(C.oid)) AS "total_size"
pg_class C
LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
WHERE nspname NOT IN ('information_schema')
AND C.relkind <> 'i'
AND nspname !~ '^pg_toast'
AND relpages > 100
ORDER BY pg_total_relation_size(C.oid) DESC
LIMIT 20;
On my trivial test install that gives me just the one user table:
relation | total_size
------+----
public.t | 3568 kB
While still showing larger catalog tables if they grow to be noticeable.
--
Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
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Re: [GENERAL] Plug-pull testing worked, diskchecker.pl failed
On 10/24/12 4:04 PM, Chris Angelico wrote: Is this a useful and plausible testing methodology? It's definitely showed up some failures. On a hard-disk, all is well as long as the write-back cache is disabled; on the SSDs, I can't make them reliable. On Linux systems, you can tell when Postgres is busy writing data out during a checkpoint because the "Dirty:" amount will be dropping rapidly. At most other times, that number goes up. You can try to increase the odds of finding database level corruption during a pull the plug test by trying to yank during that most sensitive moment. Combine a reasonable write-heavy test like you've devised with that "optimization", and systems that don't write reliably will usually corrupt within a few tries. In general, through, diskchecker.pl is the more sensitive test. If it fails, storage is unreliable for PostgreSQL, period. It's good that you've followed up by confirming the real database corruption implied by that is also visible. In general, though, that's not needed. Diskchecker says the drive is bad, you're done--don't put a database on it. Doing the database level tests is more for finding false positives: where diskchecker says the drive is OK, but perhaps there is a filesystem problem that makes it unreliable, one that it doesn't test for. What SSD are you using? The Intel 320 and 710 series models are the only SATA-connected drives still on the market I know of that pass a serious test. The other good models are direct PCI-E storage units, like the FusionIO drives. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] PostgreSQL training recommendations?
On 10/16/12 3:24 PM, Thalis Kalfigkopoulos wrote: Now I'd understand the Pg manual writers being reluctant about shifting from manual to DB-book, but I'm guessing, the manual being as well written as it is, that many of us are already using it as a learning book anyway. The official manual is a reference manual that also includes some good tutorial material. Just trying to cover that depth well, it's already so large as to be cumbersome--both from the perspective of new readers and the people maintaining it. Expecting to expand its scope even further toward the tutorial and example side is not something I'd expect to gain much traction. Every example that appears in the manual is yet another place for the documentation to break when code changes are made. And it's the same group of people maintaining both the documentation and the code. Anyone who tries to rev up adding even more docs is going to pull focus off new code. Would you like the core features to expand or to get a new type of documentation? The way things are organized right now, you can't get both. I would say that it's easier to write 400 pages of material outside of the manual and distribute them to the world than to add 40 pages to the official manual. And I say that as someone who tried wandering down both paths to see which was more productive. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Partitioning Advice
On 06/06/2012 01:07 AM, Ben Carbery wrote: The new server has a great deal more memory which I am hoping will help (shared_buffers = 8GB, total RAM 20GB), but I am looking at what might be optimal for the storage configuration. From looking at previous conversations here I am thinking of something like this.. 100GB OS (ext3) 50GB pg_xlog (ext2) 400GB pg_data (ext3 data=writeback noatime?) Hopefully this would mean the small writes can continue while a large read is going. Latency on ext3 is better on RHEL6 than earlier versions, but it's still hard to get to keep it low with that filesystem. You should consider ext4 or xfs instead if you're already running into slow periods limited by disk I/O. Large values of shared_buffers can also make write latency spikes worse, particularly when the underlying storage isn't very capable--which is likely to be the case in a VM environment. Most of the performance gain is from going from the tiny default (<=32MB) for shared_buffers to a moderate size. You'll probably get most of the performance gain setting that to around 1GB instead, and the worst case performance might improve. If you already are seeing problems on your existing server, there are two things you could do to monitor what's going on: -Turn on log_checkpoints on the server. If you see high numbers for the "sync=" section, that normally narrows your problem very specifically to the database's background checkpoints. -Watch /proc/meminfo , specificially the "Dirty:" number. If that number gets very high during the same periods the slowdowns happen at, it might be possible to make things better by decreasing the amount of caching Linux does. There's some intro material on that subject at http://notemagnet.blogspot.com/2008/08/linux-write-cache-mystery.html and http://blog.2ndquadrant.com/tuning_linux_for_low_postgresq/ (note that some of the links in that second one, to the test pgbench results, are broken; http://www.highperfpostgres.com/pgbench-results/index.htm is the right URL now) -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
Re: [GENERAL] oracle linux
On 03/28/2012 10:38 AM, Gregg Jaskiewicz wrote: They seem to claim up to 70% speed gain. Did anyone proved it, tested it - with PostgreSQL in particular ? RedHat's RHEL5 kernel is 2.6.18 with a bunch of backported features. Oracle just yanks that out and puts a closer to stock 2.6.32 based kernel in there instead. Basically the speed gain is for people who don't want to update their whole distribution, because of nonsense like "SAP etc. is only supported on RHEL5 based platforms" I think, but need the better high-speed hardware support of a newer kernel. Of course a several year newer kernel runs much faster on latest generation hardware. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Why checkpoint_timeout had maximum value of 1h?
On 03/29/2012 06:57 AM, Maxim Boguk wrote: Is there any real reason why checkpoint_timeout limited to 1hour? Just to keep people from accidentally setting a value that's dangerously high. There can be some pretty bad drops in performance if you let writes pile up for too long, once the checkpoint really does start running. In my case I have some replicas with WAL on SAS raid and PGDATA on SSD with limited write enduranceIn that case having checkpoint_timeout=10hour could reduce amout of writes on SSD by factor of 10, and increase planned ssd lifetime by the same amount. The big write endurance problem is WAL data, and you're already addressing that. Note that if nothing has been written out since the last one, the checkpoint won't actually do anything. So this 10X endurance idea might only work out on a system that's always doing something. You'll certainly get less wear; without measuring your workload better, I can't say just what the multiplier is. The other idea you should be considering, if you haven't already, is not provisioning all of the space. I would like to have ability to set checkpoint_timeout=high value and (whats even better) checkpoint_timeout=0 - in that case checkpoint happen when all checkpoint_segments were used. Is there any serious drawbacks in that idea? Is it safe to increase that limit in source and rebuild database? (9.0 and 9.1 case) You can edit src/backend/utils/misc/guc.c , find checkpoint_time, and change the 3600 value there to something higher. You will need to rebuild the whole database cluster with that setting (initdb), and moving a database cluster of files between your tweaked version to/from a regular PostgreSQL will do strange things. You can prevent that from happening accidentally by editing src/include/catalog/catversion.h ; find the line that looks like this: #define CATALOG_VERSION_NO201202141 And change it. It's just MMDDN to create a complete catalog serial number, where N is an incrementing number if more than one change is made on the same day. If you do that and increase the upper bound on checkpoint_timeout, that should do what you want, while protecting against the dangerous situation--where system catalog doesn't match the database binaries. Setting checkpoint_timeout to 0 instead won't work--it will checkpoint all of the time then. The bottom limit is 30 seconds and you don't want to touch that. It's possible to make 0 mean "never timeout", but that would require juggling a couple of code pieces around. The idea of just making the timeout big is a safer thing to consider. I'm not sure if you'll really see the gains you're hoping for, but it should be easy enough to test. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pgcon 2012
On 03/24/2012 12:46 AM, Arvind Singh wrote: Is http://www.pgcon.org/2012/?2nd a valid and certified Postgres Event or is it just a marketing strategy by Private Event Management firm There is no such thing as a "certified" Postgres event. All of the conferences in the world are run by private groups and/or companies. In addition to what attendees normally pay, there are normally a number of companies involved in sponsoring each event. In many cases the events wouldn't be feasible without that sponsorship help. Some conferences also benefit heavily from the various PostgreSQL user's groups around the world too, like the yearly European conference that you might consider too: http://pgconf.eu There are some policies for who can use PostgreSQL community resources like the postgresql.org web site to promote their event, which you can see at http://wiki.postgresql.org/wiki/NewsEventsApproval But many organizations and companies meet those guidelines, and none of them are any more or less official than the others. One helpful thing to consider when deciding which conference to attend is whether the speakers are involved in the areas you're interested in. For the database itself, a list of many frequent contributors is found at http://www.postgresql.org/community/contributors/ If you check the PGCon speaker list, you'll see many of them are also on that contributor list. That's one reason that PGCon is an excellent conference. It's also very well run by its organizers, even though they are "private event management" by your definition. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
Re: [GENERAL] Optimise PostgreSQL for fast testing
On 02/23/2012 07:16 PM, Dmytrii Nagirniak wrote: That's totally fine if PG can't beat SQLite on speed in **this particular case**. I just want to try to tune it to be as fast as it can (for **this particular case**, see my reply to Adrian). You can find all of the big tunable parameters at http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server That's as good of a general "how do I make this faster by tweaking the server" guide as you'll get. Once you've hit the big tunables--shared_buffers, checkpoint_segments, work_mem, effective_cache_size, and tweaking either synchronous_commit or fsync--there's not too much else you can do except dig into what's slow in individual queries. Only other thing that might help is running ANALYZE against the whole database after any major loading of test data, just to make sure the queries are being executed with good statistics. If you can extract the SQL from the test cases so they can be executed directly with the psql client, you could add "\timing" before them to see how long each individual query runs, to look for the long running ones. It's possible that every statement is a little slower, which would be unsurprising and not something you can really resolve if so. It could just be a small number that are being executed poorly though, in which case specific query tweaking might be possible. You might get further insight by posting the EXPLAIN ANALYZE plans of whatever the slowest single query is. More on that subject at http://wiki.postgresql.org/wiki/Slow_Query_Questions -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] fsync on ext4 does not work
On 12/19/2011 10:52 AM, Havasvölgyi Ottó wrote: PgSql 9.1.2 Debian, 2.6.32 kernel WAL filesystem: ext4 with defaults There's a pg_test_fsync program included with the postgresql-contrib package that might help you sort out what's going on here. This will eliminate the possibility that you're doing something wrong with pgbench, and give an easy to interpret number relative to the drive RPM rate. You said default settings, which eliminated "nobarrier" as a cause here. The only other thing I know of that can screw up fsync here is using one of the incompatible LVM features to build your filesystem. I don't know which currently work and don't work, but last I checked there were a few ways you could set LVM up that would eliminate filesystem barriers from working properly. You might check: dmesg | grep barrier To see if you have any kernel messages related to this. Here's a pg_test_fsync example from a Debian system on 2.6.32 with ext4 filesystem and 7200 RPM drive, default mount parameters and no LVM: $ ./pg_test_fsync 2000 operations per test O_DIRECT supported on this platform for open_datasync and open_sync. Compare file sync methods using one 8kB write: (in wal_sync_method preference order, except fdatasync is Linux's default) open_datasync n/a fdatasync 113.901 ops/sec fsync 28.794 ops/sec fsync_writethroughn/a open_sync 111.726 ops/sec Compare file sync methods using two 8kB writes: (in wal_sync_method preference order, except fdatasync is Linux's default) open_datasync n/a fdatasync 112.637 ops/sec fsync 28.641 ops/sec fsync_writethroughn/a open_sync 55.546 ops/sec Compare open_sync with different write sizes: (This is designed to compare the cost of writing 16kB in different write open_sync sizes.) 16kB open_sync write 111.909 ops/sec 8kB open_sync writes 55.278 ops/sec 4kB open_sync writes 28.026 ops/sec 2kB open_sync writes 14.002 ops/sec 1kB open_sync writes 7.011 ops/sec Test if fsync on non-write file descriptor is honored: (If the times are similar, fsync() can sync data written on a different descriptor.) write, fsync, close28.836 ops/sec write, close, fsync28.890 ops/sec Non-Sync'ed 8kB writes: write 112113.908 ops/sec -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Hope for a new PostgreSQL era?
On 12/12/2011 10:33 PM, Jayadevan M wrote: But I miss all those user-friendly trouble-shooting utilities (like Automatic Workload Repository,Active Session History etc etc) in PostgreSQL. Yes - some of them are there, but one has to search,download, configure etc. I hope many of these features will become part of the 'core' soon. That's unlikely, simply because the definition of "core" in PostgreSQL doesn't quite include the full experience of user tools like this. For example, you might see the core collect the data needed for something that acts like a AWR baseline. But you're unlikely to ever get the sort of easy baseline management+graph management tools that Oracle's Enterprise Manager layers on top of them in core. There's no place to put a GUI/web tool like that there, and I wouldn't expect that to ever change. You might see it provided as a feature to the pgAdmin tool though, or as a separate web application. The fact that there are multiple pieces of software involved doesn't have to make this hard. Ultimately the problem you're identifying is a packaging one. Something doesn't have to be in the PostgreSQL core to be packaged nicely so that you can easily install and use it. It's probably easy for you to get pgAdmin installed and working for example, and that's not a part of core. There's just been a lot more work put into packaging it than most tools have gotten so far. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
Re: [GENERAL] Hope for a new PostgreSQL era?
On 12/09/2011 08:54 PM, Greg Smith wrote: I decided about a year ago that further work on using Systemtap was a black hole: time goes in, nothing really usable on any production server seems to come out. My off-list e-mail this weekend has, quite rightly, pointed out that this cheap shot is unfair bordering on libel toward the hard working Systemtap developers. I'd like to publicly apologize for that and clarify my frustrated statement here (I'd *really* like this sort of tool available more) The main problem I've had with Systemtap is its reputation; I don't actually have any real, informed gripes about its current state. But the sort of customers I have are very risk-adverse. PostgreSQL does a good job attracting that sort of user. I'm sure we have a disproportionate number of them relative to your average open-source program. Accordingly, unless a piece of software is very trusted, it's hard for me to convince anyone to use it. (See "why Greg hates the disclaimers around the PostgreSQL contrib modules") That makes it hard for me to give Systemtap a serious spin on most of the production servers I see my hardest problems on. That's the reason behind the statement I made badly here--regardless of how much I know about it, I can't seem to get Systemtap deployed in the places I spent the most time working at. True or false, the "Systemtap is more likely to crash your kernel than DTrace" meme is out there. I think some of that is an unexpectedly bad side-effect of its open-source development. DTrace had the luxury of being hidden from the world at large until it was well formed. Whereas a lot of people saw Systemtap in a really early state, formed opinions several years ago, and the oldest of those are some of the highest ranking pages when you search for information. I just searched again today, and there's mounds of stuff from 2006 and 2007 that surely doesn't reflect the current state of things coming back from that. Systemtap didn't get a 1.0 release until September 2009. As someone who has spent a lot of time at the wrong end of the "PostgreSQL is slower than MySQL" meme, I shouldn't have just thrown this sort of criticism out there without explaining the basis for my statement. I hope this clears up what I meant. Ultimately I think we need both more tools like Systemtap and DTrace, as well as more instrumentation inside PostgreSQL, to cover all of the things people would like visibility into. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Hope for a new PostgreSQL era?
On 12/11/2011 11:39 PM, Jayadevan M wrote: At the db level, Oracle provides "Database replay" feature. that lets you replay the production server events in the development/test environment. http://docs.oracle.com/cd/B28359_01/server.111/e12253/dbr_intro.htm Won't something like this be useful in PostgreSQL? It will let us mimic the production environment load and analyze it better. There are several projects aiming at this goal in various ways: http://wiki.postgresql.org/wiki/Statement_Playback Some of the features currently under development right now will make this sort of thing easier to build into the core database. For example, the recent "Command Triggers" feature submission will make it easier to catch DDL changes as well as queries for this sort of thing. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
Re: [GENERAL] Hope for a new PostgreSQL era?
On 12/10/2011 09:28 PM, Craig Ringer wrote: One thing I think would be interesting for this would be to identify slow queries (without doing detailed plan timing) and flag them for more detailed timing if they're run again within time. I suspect this would only be practical with parameterised prepared statements where the query string remained the same, but that'd still be interesting There are actually two patches sitting in the current PostgreSQL CommitFest that allow normalizing query strings in a way that they could be handled like this even if not prepared, as part of pg_stat_statements. What you're asking for is basically a hybrid of that and auto_explain, with something smarter deciding when the explain is triggered. Interesting idea, I hadn't thought of that heuristic before. It won't be hard to do if the query normalization stuff commits. Personally I'd choose good performance monitoring over user/query priorities any day. With good perf monitoring I can script from the outside I have a lot more control, can send alerts, etc etc. Luckily for you it's hard to do it in any other order. When I think about how we'd have to validate whether query prioritization code was operating as expected or not, I imagine some extra monitoring tools really need to get built first. Might as well expose those for people like yourself too, once they're built for that purpose. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Hope for a new PostgreSQL era?
On 12/08/2011 09:48 AM, Satoshi Nagayasu wrote: For examples, I've been working on investigating PostgreSQL LWLock behaviors precisely for a few weeks, and it could not be obtained within PostgreSQL itself, therefore, I picked up SystemTap. However, SystemTap could not be used in a production system, because it often kills the target processes. :( How can I observe LWLocks in the production system? I decided about a year ago that further work on using SystemTap was a black hole: time goes in, nothing really usable on any production server seems to come out. It can be useful for collecting data in a developer context. But the sort of problems people are more interested in all involve "why is the production server doing this?", and as you've also discovered the only reasonable answer so far doesn't involve SystemTap; it involves DTrace and either Solaris or FreeBSD (or Mac OS, for smaller server hardware deployments). Since those platforms are problematic to run database servers on in many cases, that doesn't help very much. I'm planning to put that instrumentation into the database directly, which is what people with Oracle background are asking for. There are two underlying low-level problems to solve before even starting that: -How can the overhead of collecting the timing data be kept down? It's really high in some places. This is being worked out right now on pgsql-hackers, see "Timing overhead and Linux clock sources" -How do you log the potentially large amount of data collected without killing server performance? Initial discussions also happening right now, see "logging in high performance systems". I feel this will increasingly be the top blocker for performance sensitive deployments in the coming year, people used to having these tools in Oracle cannot imagine how they would operate without them. One of my big pictures goals is have this available as a compile-time option starting in PostgreSQL 9.3 in 2013, piggybacked off the existing DTrace support. And the earlier the better--since many migrations have a long lead time, just knowing it's coming in the next version would be good enough for some people who are blocked right now to start working on theirs. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Performance degradation 8.4 -> 9.1
On 11/17/2011 02:24 PM, Joseph Shraibman wrote: This query is taking much longer on 9.1 than it did on 8.4. Why is it using a seq scan? To answer that question in all cases, it's necessary to know a) the query, b) the PostgreSQL version, c) the table definitions including what indexes exist, d) the statistics collected about each column, (e) the sizes of all the indexes on any referenced table, and (f) the server parameters. Sometimes you can get useful feedback from just the first three of those, but no one call guess you why an index is or isn't being used without at least knowing the indexes that are defined. For example, it looks like the query is using an index on (eventlog_uid,jobid,type). It probably wants an index on jobid instead, but I can't tell whether you don't have one, or if one is there but it's not being used for some reason. How did you build the 9.1 system from the 8.4 data? There might just be a physical difference between the two tables. In addition to showing the table definition, two other suggestions: -Show what the better plan on 8.4 looks like, we're just seeing the slow one -Try running the individual EXISTS parts of this plan on both versions and compare. You might be able to isolate which of them is the source of the difference here. There's a longer guide to the things people tend to find useful at http://wiki.postgresql.org/wiki/SlowQueryQuestions ; this question might get a better response on the lower volume pgsql-performance mailing list too. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] : Looking for a PostgreSQL book
Achilleas Mantzios wrote: code for the cache utils pg_cacheutils is mentioned to exist somewhere but it's not there At this point the pgfincore project is the most popular way to do the work that pg_cacheutils was suggested for there. See http://pgfoundry.org/projects/pgfincore/ and http://www.pgcon.org/2010/schedule/events/261.en.html for more information. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Materialized views in Oracle
On 09/22/2011 01:34 AM, Mike Christensen wrote: If Oracle was a swimming pool, I would have those little floaty duck things on my arms. Yes, it's too bad the license to get Oracle 11g with Floaties is cost prohibitive for most companies. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] heavy swapping, not sure why
On 08/29/2011 06:12 PM, Lonni J Friedman wrote: OK, I'll reduce it to 10GB and see if there's any noticable change in performance. thanks I've never heard a report of a Linux system using more than 8GB of shared_buffers usefully, and peak performance on systems I've tested has sometimes been far less than that even. (I have one server that's stuck at 512MB!) The only report of even 10GB helping came from a Solaris test. I doubt this has anything to do with your problem, just pointing this out as future guidance. Until there's a breakthrough in the PostgreSQL buffer cache code, there really is no reason to give more than 8GB of dedicated memory to the database on Linux via shared_buffers. You're better off letting the OS do caching with it instead. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Whether the function exists a in pgsql table or not?
On 08/27/2011 01:21 PM, shuaixf wrote: In Oracle, when the table has few records, used frequently, it would use "storage(buffer_pool keep)" to keep the data in cache instead of LRU algorithm. if the function exists in a pgsql table or not? There aren't any controls to pin a table into memory in PostgreSQL. The way the database uses both its dedicated buffer cache as well as the OS one, it's hard to even define such a UI that would make sense--the database doesn't have any control or interaction with the OS cache. The mechanism for deciding what stays in the PostgreSQL cache is a bit more complicated than an LRU, while the one in the OS probably is a LRU. If the table is used frequently, it's very likely to stay in one of the two caches anyway. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant. -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Streaming Replication: Observations, Questions and Comments
On 08/24/2011 11:33 AM, Samba wrote: One strange thing I noticed is that the pg_xlogs on the master have outsized the actual data stored in the database by at least 3-4 times, which was quite surprising. I'm not sure if 'restore_command' has anything to do with it. I did not understand why transaction logs would need to be so many times larger than the actual size of the database, have I done something wrong somewhere? That's common to see. Systems that regularly UPDATE the same rows often can easily end up with a WAL stream much larger than the database. The WAL data contains enough information to replay every point in time from the base backup until the current time. That can be significantly larger than the database, which just holds the latest copy of the data. One of the biggest things that makes your WAL large are the full page writes that protect against incomplete writes. See "question regarding full_page_writes" thread happening on this list recently for details. Each time you touch a page, per checkpoint, another full copy of that page is written out. What I have to do in a lot of cases is significantly decrease the number of checkpoints in order to keep this overhead under control. The default config has a checkpoint every checkpoint_segments of work, and every checkpoint_timeout of time. That makes for a checkpoint every 5 minutes, and even more often under heavy load. If you increase checkpoint_segments a whole lot, all of your checkpoints will be based on the timeout instead. Then you can see how WAL load decreases as you increase checkpoint_timeout. I've had to set checkpoint_timeout as high as 30 minutes before on busy systems, to lower the WAL overhead. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
Re: [GENERAL] question regarding full_page_write
On 08/24/2011 11:12 AM, Martín Marqués wrote: Why aren't deltas good enough for the first 8Kb? Is there other information in the first 8Kb that make those more important? The fundamental problem is what's called a torn page. You write out a 8K page; only part of it actually makes it to disk; the server crashes. What you now have on disk is completely unpredictable. It's a mix of the old new page, but what portion of each you got, that's almost random. You cannot fix it with any delta. The only way to make sure it's back into a good state is to write an entire 8K page, a known correct copy, from some point in time. The way this is done in PostgreSQL, one of those is written out to the WAL the first time any page is touched after a checkpoint. Those become the known copy good to recover from any torn page problem. Then database recovery only has to replay activity since that checkpoint marker to fix all torn pages. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] question regarding full_page_write
On 08/22/2011 05:07 PM, Martín Marqués wrote: My question regarding your answer is, why is it important for the first page after a checkpoint and not on other page writes? The first time a page is written after a checkpoint, when full_page_writes is on, the entire 8K page is written out to disk at that point. The idea is that if the page is corrupted in any way by a partial write, you can restore it to a known good state again by using this version. After that copy, though, additional modifications to the page only need to save the delta of what changed, at the row level. If there's a crash, during recovery the full page image will be written, then the series of deltas, ending up with the same data as was intended. This whole mechanism resets again each time a checkpoint finishes, and the full page writes start all over again. One of the main purposes of checkpoints are to move forward the pointer of how far back crash recovery needs to replay from. Starting each new checkpoint over again, with a full copy of all the data modified going into the WAL, it is part of that logic. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] INSERTing rows from external file
On 08/16/2011 05:34 PM, Rich Shepard wrote: I have a file with 5500 rows formated as 'INSERT INTO (column_names) VALUES ;' that I thought I could read using psql from the command line. However, the syntax, 'psql < filename.sql' throws an error at the beginning of the first INSERT statement. Sounds like a problem with your file. Messing up CR/LF characters when moving things between Windows and UNIX systems is a popular one. Proof it works: $ psql -c "create table t(i integer)" CREATE TABLE $ cat test.sql INSERT INTO t(i) VALUES (1); INSERT INTO t(i) VALUES (2); INSERT INTO t(i) VALUES (3); INSERT INTO t(i) VALUES (4); $ psql < test.sql INSERT 0 1 INSERT 0 1 INSERT 0 1 INSERT 0 1 You might also try this: psql -ef filename.sql Which will show you the command that's being executed interleaved with the output; that can be helpful for spotting what's wrong with your input file. P.S. The fast way to get lots of data into PostgreSQL is to use COPY, not a series of INSERT statements. You may want to turn off synchronous_commit to get good performance when doing lots of INSERTs. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] backup-strategies for large databases
On 08/13/2011 05:44 PM, MirrorX wrote: at the moment, the copy of the PGDATA folder (excluding pg_xlog folder), the compression of it and the storing of it in a local storage disk takes about 60 hours while the file size is about 550 GB. the archives are kept in a different location so that not a problem. so, i dont want even to imagine how much time the uncompress and copy will take in 'disaster' scenario. If you haven't actually run this test--confirmed that you can uncompress the whole thing and get a working copy out of it again--I'd be concerned that you haven't tested your backup procedure fully. You can't really tell if a backup is good or not unless you restore it. And that process will get you a read on just how bad the recovery situation will look like if it comes to that one day. One technique I've used to accelerate the situation you're in is to always keep a real filesystem copy of the last backup somewhere. Then, rather than archive the main database directly for the base backup, you execute rsync to make that secondary copy identical to the one on the master. That should happen quite a bit faster than making a whole new backup, so long as you use the --inplace option. Once the standby copy is done, if you want a compressed archive you can then make it from the copy--with no extra load on the master. And you can then copy that again to another place too, followed by having it consume WAL files so that it eventually turns into a warm standby. If you want a true fail-over here, you're going to have to make one that is replaying WAL files as they arrive. any (file-system) solutions that keep the disks at sync like DRDB are suitable?so that the disk of the 2nd server would be at sync with the 1st. even if that works, i would still like to have a 3rd backup in the storage disks so my question remains. I doubt you'll be able to get DRDB to keep up with the volume you've got reliably. The only filesystem level solution I've seen scale nicely to handle the exact problem you have is using ZFS snapshots to make some of this easier. It's worth buying a Solaris license for some people to have that technology available. I had been hoping some of the new things in FreeBSD 9.0 would finally make it a lot more practical to consider for this sort of thing once that ships. But it looks like the issues around not supporting Intel's latest graphics drivers on recent "Sandy Bridge" servers may postpone adopting that further for me. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgres on SSD
On 08/12/2011 04:24 PM, Vick Khera wrote: 2011/8/10 Ondrej Ivanič: Ups! Well spotted Tomas! The actual values are: random_page_cost = 2 seq_page_cost = 1 With the SSD I would set these to the same value of 1. That's what I do. That probably makes sense on your RAMSAN. Sequential access on FusionIO drives is at least 3X as fast as completely random though, and worst-case it can be even slower relative to what a sequential scan can deliver. It's not the >50X difference seen on regular drives, but there's an easily measurable gap. I'm not sure if it's that the flash cells deliver stuff faster when you read a sequential series from the same cell of flash, or if it's just that there's less physical IOs happening. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgres on SSD
On 08/09/2011 07:17 PM, Ondrej Ivanič wrote: I'm about to evaluate this SSD card: FusionIO ioDrive Duo [1]. The main reason for this experiment is to see if SSD can significantly improve query performance Database size is around ~1.4TB. Main tables occupied around 1/3 (450GB, ~220mil rows) and aggregated data occupied 2/3 (900GB). All indexes are on separate table space (~550GB) ioDrive hardware is fast at executing all sorts of I/O, but it particularly excels compared to normal drives with really random workloads. For example, I recently tested them in two different systems, both head to head against regular 20 disk RAID10 arrays (Dell MD units). At sequential reads and writes, all drives were basically the same; >1.2GB/s reads, >600MB/s writes. The regular drive array was actually a bit faster on sequential writes, which is common with SSD showdowns. Your tables are pretty big; not much of them will fit in memory. If your aggregated queries end up executing a lot of sequential scans of the data set in order to compute, or for them to be utilized, you will probably discover this is barely faster on FusionIO. And you certainly could speed that up for far less money spent on other hardware. Is there a component to your workload that does a lot of random read or write requests? If so, is that chunk of the data set bigger than RAM, but small enough to fit on the FusionIO drive? Only when all those conditions are true does that hardware really make sense. For example, running a 300GB pgbench test on a system with 128GB of RAM, the FusionIO drive was almost 10X as fast as the 20 disk array. And its raw seek rate was 20X as fast at all concurrency levels. But at the same time, tests on database sizes that fit into RAM were slower on FusionIO than the regular disk array. When there's no random I/O to worry about, the slower read/write write of the SSD meant it lost the small database tests. You really need to measure your current system carefully to figure out just what it's doing as far as I/O goes to make this sort of decision. Given what ioDrives cost, if you're not sure how to do that yourself it's surely worth hiring a storage oriented database consultant for a few days to help figure it out. XEN host with 16 CPU (Intel(R) Xeon(R) CPU L5520 @ 2.27GHz). CentOS 5.6 80GB RAM Storage: some Hitachi Fibre channel SAN with two LUNs: 1st LUN has *everything* under $PG_DATA (used 850 GB) 2nd LUN has *all* indexes (index table space) (used 550GB) Make sure you do basic benchmarks of all this hardware before you start mixing even more stuff into the mix. Both Xen hosts and SANs can cause all sorts of performance bottlenecks. It's possible you won't even be able to fully utilize the hardware you've already got if it's running with a virtual machine layer in there. I have no idea how a FusionIO drive will work in that environment, but I wouldn't expect it to be great. They need a fast CPU to run well, and some processing is done in the driver rather than on the card. checkpoint_segments | 48 maintenance_work_mem | 256MB shared_buffers | 9GB wal_buffers | 50MB work_mem | 256MB checkpoint_segments should be higher, at least 64 and probably 128 to 256. shared_buffers should be lower (at most 8GB, maybe even less). maintenance_work_mem should be 1 to 2GB on a server with 80GB of RAM. There's no proven benefit to increasing wal_buffers over 16MB. This setting for work_mem can easily allow your server to allocate over 250GB of RAM for query working memory, if all 100 connections do something. Either reduce that a lot, or decrease max_connections, if you want this server to run safely. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] is max connections in a database table somewhere
On 08/10/2011 02:46 PM, Geoffrey Myers wrote:
Is the max connections value in a system table somewhere?
If you intend to do anything with the value you probably want one of
these forms:
SELECT CAST(current_setting('max_connections') AS integer);
SELECT CAST(setting AS integer) FROM pg_settings WHERE
name='max_connections';
The setting comes back as a text field when using current_setting on the
pg_settings view (which isn't a real table, under the hood it's calling
a system function)
--
Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
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Re: [GENERAL] heavy load-high cpu itilization
On 07/27/2011 04:37 AM, Filippos wrote: P.S i will send a message to the admins, to ask them to move the topic to the sub-forum of perfomance Don't do that; will just waste their time. pgsql-general is a mailing list, and the "forum" view you're seeing at Nabble is just a web interface to it. They can't move things around there because they don't really own the list; they just make a copy of all its messages. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] 100 times faster than mysql
On 07/26/2011 10:02 AM, Allan Kamau wrote: If the speed is to be measured purely (and simply) on these numbers, 186/12 yields 15.5 (or maybe 16 if your round it up or 15 if you use integer division). May be about 15~16 times faster would be more in line with numbers provided. I guess he did the math on MySQL, too. Could be worse; could have ran into http://bugs.mysql.com/bug.php?id=33704 which, as you can see, is totally not a bug. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] heavy load-high cpu itilization
On 07/26/2011 01:47 PM, Filippos wrote: we have a dedicated server (8.4.4, redhat) with 24 cpus and 36 GB or RAM. i would say that the traffic in the server is huge and the cpu utilization is pretty high too (avg ~ 75% except during the nights when is it much lower). i am trying to tune the server a little bit to handle this problem. the incoming data in the database are about 30-40 GB /day. Well, the first question is where the CPU usage is coming from. There are two basic schools of thought here: 1) Use real-time monitoring utilities like "top -c" and see what is gobbling time up. It's possible to miss what's happening, but if you're at 75% a large chunk of the day that doesn't seem likely. 2) Set log_min_duration_statement and the other logging parameters; analyze the resulting log files to see where the CPU time is going. You seem to be focused on the background writer and its checkpoint process right now. That cannot be the source for high CPU usage; at most it could fully use one of your 24 cores. You should fix wal_buffers to a reasonable value regardless, but your problem is not in that area. Importing 30-40 GB/day is extremely difficult to do in PostgreSQL. My guess is that most of the server time is spent running the data import process itself--even COPY, the most efficient way to get data in, is very CPU intensive. The second layer of problems here that can increase CPU usage come from autovacuum taking up a lot of resources to run, which it will do all the time given this volume of activity. And there's always the possibility that the queries you're running against the data are just taking a long time to execute. Another layer of problems in this scenario you'll hit eventually is that you'll need to have your tables partitioned in order to prune old data out efficiently. Presumably you can't keep up with that rate for very long before you have to start dropping older data, and that's really hard to do efficiently unless you've used partitions. P.S. You should upgrade to PostgreSQL 8.4.8 as soon as possible. There is a bug in autovacuum that's been resolved as of 8.4.6 that you are very likely to run into: http://www.postgresql.org/docs/8.4/static/release-8-4-6.html P.P.S. The pgsql-performance list would be a more appropriate place to have this discussion at. Some of the people who provide good input over there on topics like this don't read pgsql-general, too many messages on this list for them. are there any suggestions what i can do to tune better the server? i can provide any information you find relevant for the configuration of the server, the OS, the storage etc There's a chapter on each of these in my PostgreSQL performance book, and I'm not aware of any other resource that takes on all of these topics usefully. If you're trying to keep up with this volume of data, buying a copy of that should repay itself in time savings--where you can look something up rather than trying to figure it out from scratch--about once every week. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Building an home computer for best Poker Tracker performance
On 07/20/2011 02:14 AM, Stuart Cooper wrote: Nice to see some poker being discussed on this list. Much more reputable than Stock Trading. If the casinos did something like what traders call "front-running", they'd all be put in jail the next day for cheating their customers. Just a day in the life of many trading firms though. Opponent: AT Stuart the LuckyBum: 88 Flop: ATT Myself and opponent checked that flop, Since they decided to slow-play after flopping three of a kind in a limit game, they deserved to lose the hand. Never should have let you see the next card for free; they got greedy and paid for it. Opponent was pretty surprised to lose, after his third river raise I began to consider he might even have hold TT for better quads than mine. I know that sinking feeling well. My last one involved hitting an Ace-high flush on the river, only to see a fourth of that suit appear on the river. When those ragged cards sorted in my head for the first time, I realized my suddenly excited opponent must have just finished his straight flush. Oops. At least I made him pay to see the winning card. I used to average around 150 hands per hour; that rate can show you one of these 990:1 shots every 7 hours of play. Leaves one with a healthy respect for the sharp pointy bit on the end of the bell curve, after you've been stabbed with it a few times you start to remember it's there. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Building an home computer for best Poker Tracker performance
Bruce Momjian wrote: Greg, tell me you didn't get involved with Postgres because of Poker Tracker. :-O :-) Nah, both came out of my working on stock trading systems. I just wrote a bit more about this whole subject at http://blog.2ndquadrant.com/en/2011/07/pushing-allin-with-postgresql.html if anyone would like to see what the actual queries look like against the Poker Tracker database. One of the things you have to come to grips with when writing a trading system is that you can never assume something won't happen just because it's really unlikely. Traders have adopted the term "black swan" to talk about these sort of completely unexpected things that destroy their bankroll. On-line poker is quite good at teaching you this lesson quickly, whereas traders might go a decade or more before encountering a similarly unlikely event. Lose an ace-high flush to a straight flush; lose 4 of a kind to a higher 4 of a kind; rare, but I watched them both happen to me on multiple times when playing. If I'd bet so much that I'd have been wiped out by either event, even though I was *sure* I would win that hand, I'd have been done for. And a couple of times, I've seen the most rare thing of all: the initial 3 cards come out, and I have a hand where I can only lose if the other player gets the exact two cards they need. For example: Greg: KK LuckyBum: QQ Flop: K82 The only way the other player can win here is if the remaining cards are QQ, giving them 4 of a kind over my full house at the end of the hand. Assuming no other players are involved, that's 45 cards left in the deck, and the odds of them getting one of those two followed by the other are 2/45 * 1/44. And yet I've lost this sort of 990:1 long shot multiple times. It definitely gives you a better gut feel for "gambler's ruin", one that translates back into stock trading--and into thinking about how to really achieve high-availability for a computing system, too. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Programmer ( Postgres), Milwaukee - offsite-Remote - onsite
On 07/18/2011 05:58 PM, MS Rao wrote: *Programmer ( Postgres), Milwaukee -- offsite-Remote - onsite* We are looking for Postgres skilled programmer with the following skills: Job listings are not appropriate for the pgsql-general mailing list. Please direct them to the pgsql-jobs list in the future instead: http://archives.postgresql.org/pgsql-jobs/ In addition to that being the policy here, using that list instead means that we can block people replying to the whole list with their resumes and similar details they didn't mean to make public (which does happen). -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD
Re: [GENERAL] announcements regarding tools
On 07/19/2011 09:00 PM, Bruce Momjian wrote: We have the same problem with people posting to pgsql-jobs where they don't mention the location of the job. I usually email the people privately about this. Used to have. pgsql-jobs is now moderated because the junk posts were starting to outnumber the real ones. I just ejected an off-topic J2EE job posting earlier today that would have made it through until this recent change. Jobs that are too vague on details to be useful should get similarly bounced back to the poster, to be reworked before they'll be accepted. It's impractical to do the same thing for something as complicated as platform support, at least using negative reinforcement, on the more controversial and popular announce list. The list of rules at http://wiki.postgresql.org/wiki/NewsEventsApproval is already too big for moderators to be expected to enforce them all correctly in every case; adding more isn't likely to do anything useful. The best I think we could do here is update that page to encourage people to list this information for their own benefit, and then have some regular announcers lead by example, by including the sort of info Scott is asking for into their messages. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Building an home computer for best Poker Tracker performance
ting for 4 to 8 of them probably. Two final notes: -Make sure to follow the basic database tuning guidelines at http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server to get your configuration in the right area. I think your workload might really benefit from turning off synchronous_commit in particular, so try both settings there. It won't kill you to lose a hand or two of history if you disable that, and the speed improvement could be large. -If you want to start up a discussion about optimizing your server, that would be better done on the pgsql-performance list than this one. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] How to create "auto-increment" field WITHOUT a sequence object?
On 06/30/2011 03:01 PM, Dmitry Koterov wrote: It may not be "fully" gapless. The main cause is to keep uniq_id as low as it could be to not to exhaust 10 values too fast. I think solutions with addition tables look too complicated for this case, is there a possiblilty to not to use an additional table? You may think it's possible to build a gapless design that is less complicated by writing some application or server code to enforce it. You've already tried this and learned that it's much harder than it seems. Doing this correctly without causing timeout and deadlock issues is a hard problem. Meanwhile, generating a gapless translation table that only includes things that have been committed is easy, and you're not likely to run into really strange and unexpected bugs in that implementation later. Given those are the two situations you're comparing here, I would say using the extra table is less complicated in every way. Sure, you're adding another table, but the process happening against it is really easy. The alternative doesn't have the extra table, but that doesn't make it less complicated. Complexity needs to consider how difficult a program is going to be to debug and maintain. And in those areas, making a single table gapless is quite complicated. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Setup postgres with automatic table and user intitiallisation
On 06/28/2011 05:34 AM, Lodron, Gerald wrote: I have the problem that when i run my batch i always have to let the user type in the password, thats nasty... I also tried PGPASSWORD but it does not work, it always prints that it is the wrong password... I'm not sure if setting PGPASSWORD in a Windows batch file is going to work correctly...have you tried creating a .pgpass file instead? http://www.postgresql.org/docs/9.0/interactive/libpq-pgpass.html Another option to consider is swapping out the default pg_hba.conf file with one that trusts local users fully. You could put that in place, signal the server to reload its configuration, execute your setup script, then put back a properly secured file and reload again. There will be a brief window where any local user could gain access to the database server as a superuser that way. But if you were that worried about locking down security, you probably wouldn't be deploying an auto-installer on Windows in the first place. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] rationale behind quotes for camel case?
On 06/28/2011 12:09 PM, dennis jenkins wrote: Example: Suppose that I have a table called "foo" and another table called "barBiz" (or "bar_biz" if you prefer). Further, both of these tables have a "serial" primary key. Now I want to create a third table that represents a many-to-many relationship between "foo" and "barBiz". So far I have been keeping compound-noun table names in camel case, but mapping tables separate the base table names with underscores. Thus the table name would be "foo_barBiz". The options are basically: 1) Use case to help sort this out 2) Use lots of underscores and cope with the ambiguity 3) Pick something to put in the middle to represent relationships between things, to make them less ambiguous. You might name this foo_to_barbiz or the compact but expressive foo2barbiz as two examples. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Multi-tenancy in Postgres
On 06/28/2011 05:45 PM, Rob Sargent wrote: I think Greg might be forgetting that some of us don't always get to choose what we work on. I was in a shop that decided to go with multi-tenancy for reason both technical and um, er envious. There are certainly successful deployments of multi-tenant PostgreSQL out there, ones that make sense. What I was trying to communicate is that the particular variation proposed by this academic paper doesn't seem the right direction for PostgreSQL development to head in to me. This project is stubborn about resolving the problems people actually have, and the ones the paper tries to solve are not the ones I've seen in my own experiments in multi-tenant deployments. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Multi-tenancy in Postgres
Emrul Islam wrote: I've just read through a paper here: http://www.edbt.org/Proceedings/2011-Uppsala/papers/edbt/a12-schiller.pdf about multi-tenancy. They used Postgres for their work and while it is academic and would need further work I'm just wondering if anyone in the Postgres team is looking at implementing some of the functionality described? They seem fuzzy about what actual businesses who implement multi-tenant environments, such as hosting companies, actually want and find missing in PostgreSQL right now. They've tried to solve a problem I never would have considered interesting in the first place. On the "Shared Machine" side of things, we find complaints like how individual PostgreSQL instances use too much power. See "Latch implementation that wakes on postmaster death", currently under development aimed at 9.2, aimed right at kicking that one around. This "Shared Table" approach they spend so much time worrying about and improving? No one cares about that except companies hosting a single application on their giant box. This idea that there are large number of tenants running the same application, but whom need to be isolated from one another in some way, is not the normal state of things. Yes, it happens on the big servers at Salesforce.com who all run the same application; that is not a common situation however. What the hosting companies actually want from PostgreSQL is a good implementation of "Shared Process". One database install, every tenant gets their own schema, tables and are expected to use some resources. You can do this right now; I believe the infrastructure at Heroku is built that way for example. How do the ideas in this paper actually solve the problems they're seeing with that approach? I don't know for sure, but I don't see anything exciting there. I makes me kind of sad when people put a lot of work into doing a good job on a problem that doesn't really matter very much in the real world, and that's the overwhelming feel I get from reading this paper. Advanced schema inheritance stuff? Don't care. Providing query cost constraint limits for individual tenants? Now that's a useful problem to talk about, one that people deploying multi-tenant databases are actually being killed by. And discussing aspects of that problem does flare up among the PostgreSQL developers regularly. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] rationale behind quotes for camel case?
fluca1...@infinito.it wrote: first of all I'm not expressing any critique against the use of quotes for identifier expressed using camel case. However a lot of new postgresql users seems to be unhappy with the use of quotes for camel case identifiers, so I'd like to know what is the rationale behind it. I mean, is a feature of the partser or is a need for it to work? Anyone can point me to an explaination? The need for quoting and use of CamelCase are only vaguely related to one another. There are situations other than CamelCase where quotes are needed, and using CamelCase doesn't necessarily require quotes. If you follow the end of the documentation at http://www.postgresql.org/docs/9.0/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS you'll find this explanation of what the quoting issue is all about: Quoting an identifier also makes it case-sensitive, whereas unquoted names are always folded to lower case. For example, the identifiers FOO, foo, and "foo" are considered the same by PostgreSQL, but "Foo" and "FOO" are different from these three and each other. (The folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, which says that unquoted names should be folded to upper case. Thus, foo should be equivalent to "FOO" not "foo" according to the standard. If you want to write portable applications you are advised to always quote a particular name or never quote it.) So saying you're unhappy with the need for quoting of mixed-case items isn't going to get you very far, as the behavior here is all tied up with the trivia of the SQL spec. The specification intends that anything other than all upper-case naming requires quoting, and PostgreSQL turns that around to say that everything goes to lower-case by default. It's actually possible to leave out the quoting if you really want to, but as advised here you have to be consistent about it. Once you've created something with a fully quoted name, instead of letting it "fold" the name to all lower-case, you must continue to reference it that way in the future. There's nothing stopping you from just never quoting anything though. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD Comprehensive and Customized PostgreSQL Training Classes: http://www.2ndquadrant.us/postgresql-training/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Oracle / PostgreSQL comparison...
On 06/23/2011 10:28 PM, Stephen Frost wrote: I love how he finishes with the claim that Oracle "keep their finger on the pulse of where IT is headed", right after admitting that their client is actually a huge piece of junk. Oracle is able to keep their finger on the pulse of their customers, because they have their hands where they can firmly squeeze their...uh, wallets. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Help making tablespaces work for my application
On 06/23/2011 09:37 AM, Natusch, Paul wrote: I have an application for which data is being written to many disks simultaneously. I would like to use a postgres table space on each disk. If one of the disks crashes it is tolerable to lose that data, however, I must continue to write to the other disks. Tablespaces are not useful for implementing this idea yet. 1. There is a single WAL log for the entire cluster, located in the pg_log subdirectory. If the disk containing the pg_log file crashed, does that mean the system would come to a halt. Is there anyway to distribute this data so that WAL is located on the same media as the table space? An alternative would be to use raid with the disk that stores the pg_log subdirectory but that adds cost to the system. Loss of the pg_xlog subdirectory and the WAL contained in it normally results in catastrophic database failure. Recommended practice is to use a RAID-1 volume to make odds of that failure lower. 2. If #1 was solved by using the raid approach, what happens if one of the disks containing one of my table spaces crashes. At some point postgres will want to write the data from the WAL file to the crashed (unavailable) disk.Will postgres will be blocked at this point? Is there some way to notify postgres that a specific disk is no longer available and that the entries in the WAL for this disk should either be purged or ignored? ( I'm willing to "throw away" the data on the crashed disk). PostgreSQL can't be expected to operate sanely when faced with the loss of an individual tablespace. It may be possible to recover from it, but you'll be doing something it's not designed to handle, and that effort may not succeed. Note that any tablespace failure is likely to require taking down the database to repair the involved tablespaces, so you're likely to have downtime between a component failure and when you notice to take action. The database really does not like having tablespaces just go away in the middle of operations. PostgreSQL 9.1 (not released yet, currently in beta) includes a new feature called "unlogged tables" that might make this sort of deployment possible. If you created a tablespace for disposable data and put an unlogged table onto it, loss of that tablespace would me much less likely to cause a problem. So long as you recreated a new space for the unlogged table after restarting, you could probably recover having only lost the data on the crashed disk in this situation. Clearly using raid on all of the disks would be a solution, but that is cost prohibitive. On a cheap server I can easily RAID-1 mirror a pair of drives on Linux using software RAID, and individual drives are $50 to $100 each. If your data isn't worth that much, And even that's not enough to really make me feel secure about the data--you really need to keep another copy around as a backup, too. You can treat your data as disposable and expect to lose it when any single component fails, or you can include some good redundancy practices in the design to reduce odds of a failure. There really isn't really a good solution providing partial protection in the middle of those two. -- Greg Smith 2ndQuadrant usg...@2ndquadrant.comBaltimore, MD PostgreSQL Training, Services, and 24x7 Supportwww.2ndQuadrant.us "PostgreSQL 9.0 High Performance":http://www.2ndQuadrant.com/books
Re: [GENERAL] Oracle / PostgreSQL comparison...
On 06/23/2011 10:28 PM, Stephen Frost wrote: Next, PG doesn't even use the same basic technology as Oracle regarding how transaction isolation and versioning works. Oracle using rollback segments to store 'old' rows in, while PG uses a Multi-Version Concurrency Control (MVCC) system. They're fundamentally different things, so the notion that PG is somehow a 'reverse engineered' Oracle is complete bunk. I stole some inspiration from this comment for my own response, which I just posted to the site. I'll save a copy here in case the author becomes so embarrassed by his mistakes he deletes it: The idea that PostgreSQL is reverse engineered from Oracle is ridiculous. Just a look at the vast differences in the MVCC implementation of the two; Oracle's redo logs vs. PostgreSQL WAL are completely difference designs. As for there being no unique features in PostgreSQL, that's completely wrong too. A good example is how deep the transactional DDL <http://wiki.postgresql.org/wiki/Transactional_DDL_in_PostgreSQL:_A_Competitive_Analysis> features go--Oracle has started to catch up recently, but PostgreSQL still has a lead there. The ability extend the type system and indexes with your own custom items are also better in PostgreSQL than any other database. This is why the PostGIS add-on (built using the type extension facility) is busy displacing installations of the weaker Oracle Spatial at installations all over the world right now. As for support, there are half a dozen companies in the world you can buy PostgreSQL support from at a fraction of the rate Oracle charges for it. I routinely fix bugs in the database itself within hours of report for my customers, as part of a service contract, which is an option on top of the free community support. Because PostgreSQL is open-source, there are multiple vendors available who provide this service. With Oracle as a closed source product, there can only be one who is capable of offering this quality of support. And that single source vendor has quite a history of squeezing as many dollars out of its customers as its can. Since there is choice among PostgreSQL support companies, you'll never get into that position with it. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
Re: [GENERAL] Help needed with PostgreSQL clustering/switching from MySQL
On 06/21/2011 10:00 AM, Vick Khera wrote: Postgres has nothing quite like the MySQL cluster mode with NDB. You will have to re-think your solution if you want to use postgres to distribute your queries and data across multiple servers. The closest thing to a NDB cluster in PostgreSQL is using PL/Proxy to split data across multiple nodes. Both use similar hash-key methods to distribute things across more than one system, and you can run queries that return a combined set of results bigger than any single node could have handled. But even that's extremely different from NDB as far as what the interface for executing queries is like. Vick is absolutely right here: asking about whether PostgreSQL solves the very specific problems that MySQL NDB has isn't even the right question. The two don't compare directly at all; different replication approach, different node distribution approach, different query approach. You need to return to basics instead: what is the actual business and/or technology need that has to be solved? From that there may be a PostgreSQL solution that makes sense, using its replication and query distribution mechanisms. But it's extremely unlikely that will look like a NDB cluster at all, and therefore very unlikely to have the same problems at all. You'll get a whole new mystery set instead! One of the most common mistakes I see people make when architecting database systems is assuming they have to use one of these really complicated sharded approaches to make their server perform well. Unless you have a massive database or extremely high write volume, it's way more trouble than it's worth to go through distributing writes onto multiple nodes. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Tuning for a tiny database
On 06/21/2011 01:49 AM, CSS wrote: Some raw numbers: We're only looking at a total of about six tables in one db. In total there are going to be well under 10,000 records in ALL tables. That might increase to at most 100,000 in the next few years. Our raw DNS queries/second tops out around 50 qps over three distinct servers. Keeping in mind that PowerDNS is doing heavy caching, we should never really see more than a few db queries per second. I doubt you really need to do any tuning for this scenario. I would set shared_buffers to a modest value--maybe 256MB--and stop further tuning until there's some evidence it's necessary. If presented with the same problem but with the much harder twist "I need to support >10,000 queries/second", I would recommend: -Populate a prototype with a representative amount of data -Measure the database size -Set shared_buffers to that -Whenever the database is restarted, construct a series of queries that forces all the data used regularly into the database's cache -Use pg_buffercache to confirm what's in there is what you expect Getting all the data into cache is sometimes harder than expected. Some optimizations in PostgreSQL keep it from caching large amount of tables when you do a sequential scan of the contents, as one example that complicates things. But if you get to where this is necessary, building such a tool isn't difficult, and there are some projects out there that address this particular need: filling the cache back up with relevant data after restart. This is the main one: http://pgfoundry.org/projects/pgfincore/ http://www.pgcon.org/2010/schedule/events/261.en.html -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] While converting the Master to Standby , FATAL: timeline 2 of the primary does not match recovery target timeline 1
On 06/18/2011 05:22 AM, Nithya Rajendran wrote: We are doing Master/Standby setup, We unplugged the master server, then the standby server became the master (by creating trigger file). While converting the old master to new slave server, We are getting the following error in old master, You can't make a system that used to be in the replication set join it again the way you're trying to do. The timeline feature you're seeing errors from is there to keep you from making mistakes like this. Even though this system used to be the master, it still needs to go through the full procedure for creating a new standby: start a backup, sync all the files from the master, send new archive WAL data over. The fact that this used to be the master doesn't change that. If the database is large, it may be worthwhile to use rsync and features such as its "--inplace" feature to make the copy from new master -> old master faster. Since it has older copies of the files, the copy can go faster than one to an empty system would take. But you can't just convert the old master to be a standby of a new master. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
Re: [GENERAL] Another RAID controller recommendation question
On 06/18/2011 02:46 AM, David Boreham wrote: We're looking to deploy a bunch of new machines. Our DB is fairly small and write-intensive. Most of the disk traffic is PG WAL. Historically we've avoided RAID controllers for various reasons, but this new deployment will be done with them (also for various reasons ;) If the traffic is heavy on WAL, avoiding RAID controllers isn't a great practice. They're by far the best way possible to speed that up. http://www.newegg.com/Product/Product.aspx?Item=N82E16816101339 manufacturer page : http://www.supermicro.com/products/system/1U/6016/SYS-6016T-URF4_.cfm?UIO=N This a solid basic server model. The Intel 5520 chipset they're built on is nice and fast if you load it up with a bunch of RAM. these boxes have a proprietary controller slot, with these cards: http://www.supermicro.com/products/nfo/UIO.cfm#Adapters specifically this LSI-based one which seems to be the newest/fastest, with BBWBC: http://www.supermicro.com/products/accessories/addon/AOC-USAS2LP-H8iR.cfm I don't really like the whole proprietary controller slot thing if it can be avoided. We seem to have a lot of customers buying from Dell recently, and it's partly because they've made it pretty straightforward to swap out their PERC controller. That makes troubleshooting a broken server easier, spare parts are simple to manage, lots of advantages. You almost need to stock your own spares for things like the RAID cards if they're these propriety slot ones, because you're unlikely to find one in an emergency. That said, the card itself looks like plain old simple LSI MegaRAID. Get the battery backup unit, check the battery and cache policy to make sure they're sane, and learn how to use megaci to monitor it. Fast and generally trouble free after that initial setup time investment. These machines are operated in a lights-out mode, and will handle heavy constant load (hundreds of write txn/s) with 15K SAS drives in a RAID-1 setup (2 drives, or 2 + 2 with data and WAL split between spindle groups). If you can try to measure the exact ratio of database to WAL traffic here, that might help guide which of these configurations makes more sense. Hard to answer in a general way. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] 2 questions re RAID
On 06/17/2011 01:02 PM, Scott Ribe wrote: 1) Is my impression correct that given a choice between Areca& Highpoint, it's a no-brainer to go with Areca? I guess you could call Highpoint a RAID manufacturer, but I wouldn't do so. They've released so many terrible problems over the years that it's hard to take the fact that they may have something reasonable you can buy now (the 43XX cards I think?) seriously. And, in further digging, I discover that gh is an option for me. Anyone got comments on these? (I notice that they use ultracapacitor/flash to protect cache...) Atto is so Mac focused that you're not going to find much experience here, for the same reason you didn't get any response to your original question. Their cards are using the same Intel IO Processor (IOP) hardware as some known capable cards. For example, the ExpressSAS R348 is named that because it has an Intel 348 IOP. That's the same basic processor as on the medium sized Areca boards: http://www.areca.us/products/pcietosas1680series.htm So speed should be reasonable, presuming they didn't make any major errors in board design or firmware. The real thing you need to investigate is whether the write cache setup is done right, and whether monitoring is available in a way you can talk to. What you want is for the card to run in write-back mode normally, degrading to write-through when the battery stops working well. If you don't see that sort of thing clearly documented as available, you really don't want to consider their cards. 2) I understand why RAID 5 is not generally recommended for good db performance. But if the database is not huge (10-20GB), and the server has enough RAM to keep most all of the db cached, and the RAID uses (battery-backed) write-back cache, is it sill really an issue? You're basically asking "if I don't write to the database, does the fact that write performance on RAID5 is slow matter?" When asked that way, sure, it's fine. If after applying the write cache to help, your write throughput requirements don't ever exceed what a single disk can provide, than maybe RAID5 will be fine for you. Make sure you keep shared_buffers low though, because you're not going to be able to absorb a heavy checkpoint sync on RAID5. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] how to find a tablespace for the table?
On 06/17/2011 06:50 PM, hyelluas wrote: I'm looking into pg_tables view and only one tablespace is displayed is pg_global. All my tables are created in my custom tablespace and that column is empty for them. I'm not sure what's wrong here, but the query you are trying to use to decode this information doesn't look quite right. pg_tables is just a regular query; here is its source code: CREATE VIEW pg_tables AS SELECT N.nspname AS schemaname, C.relname AS tablename, pg_get_userbyid(C.relowner) AS tableowner, T.spcname AS tablespace, C.relhasindex AS hasindexes, C.relhasrules AS hasrules, C.relhastriggers AS hastriggers FROM pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) LEFT JOIN pg_tablespace T ON (T.oid = C.reltablespace) WHERE C.relkind = 'r'; I think that if you start with this and try to experiment from there, you may be able to figure out what's going on here a little better. This connects up the main relevant tables in the right way. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgres performance and the Linux scheduler
On 06/16/2011 02:10 PM, Simon Windsor wrote: Can the performance of Postgres be boosted, especially on busy systems, using the none default DEADLINE Scheduler? Some system reports moderate improvements in throughput, and sometimes larger ones in worst-case latency, when switching from the default to the deadline scheduler. Others report best performance with the noop schedule. Changes here are not effective in a lot of cases though. I wrote an article for the first issue of PostgreSQL Magazine that mentions this, as the last of the most useful things you can tweak on Linux; that's available at http://pgmag.org/ -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
Re: [GENERAL] PostgreSQL 9.0 or 9.1 ?
On 06/16/2011 10:06 AM, Achilleas Mantzios wrote: Till the end of July i must have finished all the migration to the new versions. So i am asking what would be better from your perspective to do? Go for 9.1? or stick to 9.0 and try to deploy it and take the most out of it? When is a stable (release) version of 9.1 be available? Has any one faced any issues migrating from 9.0 to 9.1 I would place odds at about 1/3 that 9.1 will be available by the end of July. But you will still need to do testing of your application first before deploying onto that version. Realistically, even the earliest of 9.1 adopters is unlikely to launch before August. As such, there's not very much experience about the migration available yet, either. A large number of the new features in 9.1 aim at making certain types of development easier. The must-have features I am hearing demand for from my customers (who admittedly care more about replication and performance features than most), such that they are postponing some deployments until 9.1 ships because 9.0 just doesn't do what they want, are: -Synchronous replication -Support for MIN/MAX queries against partitioned tables -Feedback mechanism to reduce query conflict resolution when using Hot Standby -Much improved monitoring for replication and Hot Standby queries I'd suggest you take a look at the 9.1 release notes and beta announcement: http://www.postgresql.org/about/news.1313 , http://www.postgresql.org/docs/9.1/static/release-9-1.html And if you don't see a major compelling reason to wait for 9.1, some feature in that list that makes your life a lot easier, you really should just deploy 9.0 and move on. The most critical thing fixed in 9.1 development that may apply to what you're doing--some bug fixes to pg_upgrade--have all been backported to 9.0 now. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] You could be a PostgreSQL Patch Reviewer!
On 06/15/2011 04:49 PM, Josh Berkus wrote: You do not have to be a C coder to be a patch reviewer. Pretty much all you need to know is: - how to checkout PostgreSQL from Git - how to build PostgreSQL from source - how to apply a patch And you don't even really need to know these things well. If you're on a UNIX system that has git installed, here's a complete example of how to build a custom PostgreSQL that includes a patch for review: cd git clone git://github.com/gregs1104/peg.git export PATH="$HOME/peg:$PATH" mkdir pgwork peg init test cd pgwork/src/test patch -p1 < ~/mytest.patch . peg build psql Just substitute the name of the patch you're using in the "patch" step here, and if it applies correctly (one of the first things patch review intends to test for) you're done. This does a local installation of PostgreSQL into a tree under your home directory, with reasonable defaults for all of the parts related to downloading the source code and compiling it. See the documentation for the peg utility at https://github.com/gregs1104/peg for more examples -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] determine client os
On 06/13/2011 07:04 AM, Sim Zacks wrote: I didn't see a function for this, but is there a way in a postgresql query to determine the client OS? A PostgreSQL client application is something that speaks a correct protocol to the server. The server has no concept of what the client is other than the fact that it speaks a particular version of its language. It knows the IP address and port number it connected on, but that's basically it. So the idea of a query determining the client OS doesn't make sense; all it knows is what the client tells it, and the information required to connect to the server and execute queries does not include any such details. If you want information about a client to make its way into a statement run on the server, you have to drive that from the direction of the client you're using yourself. If your client is psql for example, you might pass client-side information into the program by using the -v/--set/--variable substitution mechanism, possibly combined with the SQL interpolation facility of psql. But if your client program is in another programming language, you'll have to use some facility in it to fill in this information. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Write performance on a large database
On 06/09/2011 08:15 AM, Håvard Wahl Kongsgård wrote: Hi, I have performance issues on very large database(100GB). Reading from the database is no problem, but writing(or heavy writing) is a nightmare. I have tried tuning postgresql, but that does not seem to improving the writing performance. To improve the write performance, what are my options? Well, technically you have performances issues on a medium sized database. The simplest answer to your question is "buy a server with 96GB of RAM". If it's still possible to get a server that holds your entire database in memory for a moderate investment, it's really not large yet. There are many free guides that discuss various aspects of write performance and tuning around them, some of which are excerpts from my book which goes over all of this territory: http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server : Covers general server tuning http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm : Discusses the exact way checkpoints work and how people commonly tune them http://wiki.postgresql.org/wiki/Reliable_Writes : all four of the references there cover this area. http://projects.2ndquadrant.com/talks : "The Write Stuff" presentation goes over some of the limitations people run into with high write volume applications. I'd suggest taking a look at those. If you want to talk more about this afterwards, start a new discussion on the pgsql-performance list with some of the information recommended at http://wiki.postgresql.org/wiki/SlowQueryQuestions : disk controller and disk info, PostgreSQL version, and database server configuration all have a lot of impact here. The contents of pg_stat_bgwriter would be interesting too. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Regular disk activity of an idle DBMS
On 05/29/2011 02:42 PM, Andrej Podzimek wrote: I identified the most active process, at least twenty times more active than any other process on the system: postgres 3086 0.1 0.0 34688 2584 ?Ss 03:11 1:16 postgres: stats collector process So it's the statistics collector. However, there does not seem to be any database activity at all. I tried looking at the numbers returned by this query: select datname, tup_returned, tup_fetched from pg_stat_database ; Nothing changes there. When OpenFire, Courier-MTA and Apache are restarted, a few numbers change, but othrewise they remain unchanged pretty long. There is no obvious activity that could trigger a disk write 20 times a minute... There are things that the statistics collector might be updating that don't show up in the pg_stat_database totals. It aims to write updates approximately every 500ms, so your write rate sounds normal. The expectation is that the operating system is actually caching most of those, so that the actual load on the system is minimal. So it sounds like you've identified the cause here, and it is normal, expected activity. One thing that can cause statistics overhead to be higher than it should be is a larger statistics file than is strictly necessary. We hear reports of those sometimes, I've never been completely clear on all of the possible causes that make this happen. But running "select pg_stat_reset();" should clear that out and start fresh again. That will sometimes eliminate situations where the I/O seems larger than it should be for people. If you do that, and there's still activity going on, there's no easy way to fix that. As mentioned in http://www.postgresql.org/docs/9.0/static/monitoring-stats.html , it's possible to change PGSTAT_STAT_INTERVAL at server compile time to make it write statistics less frequently. There's no easier way to adjust that though. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Regular disk activity of an idle DBMS
On 05/28/2011 11:02 AM, Andrej Podzimek wrote: after configuring a new home server with PostgreSQL 9.0.4, I observe some regular disk activity, even though the server is completely idle (disconnected from the network, no users but one logged in). There are very short write bursts once in about 3 seconds. There are a couple of things that can cause unexpected disk activity: -autovacuum running in the background. Setting log_autovacuum_min_duration may help you determine when this is happening. -checkpoint activity. Turning on log_checkpoints, as well as looking for changes in the pg_stat_bgwriter view, may help explain if this is the case. -Hint bit updates. Even if you are only reading from a table, in some situations write activity can be generated. See http://wiki.postgresql.org/wiki/Hint_Bits for more information. -Statistics collector updates. If the one logged in user is doing anything at all, they might be generating something here. Figuring out if the writes are happening from a regular PostgreSQL process, or if they are happening via the background writer, might also be useful here. Saving the output from "top -b -c" can be useful for this. The iotop command is very helpful for tracking down this sort of problem too. The background writer process, which also handles checkpoints, will have the same process ID once it's started. So will the statistics collector. If you track I/O to one of those two, it should narrow possible causes quite a bit. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SELECT COUNT(*) execution time on large tables (v9.0.4-1)
On 05/27/2011 12:41 PM, Carlos Sotto Maior (SIM) wrote: I have browsed catalog tables, digging for a real time Row.count but so far did not find any. See http://wiki.postgresql.org/wiki/Slow_Counting which shows you where the one system count estimate is at, as well as suggesting links to where you can find alternate approaches here. If you need an exact count and can't afford to generate a full query to find one, some sort of trigger-based approach is likely where you'll need to go. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Inspecting a DB - psql or system tables ?
On 05/27/2011 01:24 PM, Andre Majorel wrote: While parsing the output of psql is cumbersome, accessing the system tables seems more likely to break whenever a new version of PostgreSQL comes out. I think you have this backwards. If there's a change in this area big enough to justify changing the format of the system tables, odds are the text output from psql is going to be changed too. psql gets tweaked to display information better more often than the internals are altered. Approaches you can take here, from most robust in the face of changes to most fragile, in my mind are: 1) Use information_schema. If all the info you need is in here, great; it may not be though. 2) Use the system catalog data directly 3) Parse text output from psql. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] max_connections proposal
On 05/25/2011 10:58 PM, Craig Ringer wrote: max_connections = 100 # (change requires restart) # WARNING: If you're about to increase max_connections above 100, you # should probably be using a connection pool instead. See: # http://wiki.postgresql.org/max_connections # # Note: Increasing max_connections costs ~400 bytes of shared memory # per connection slot, plus lock space (see max_locks_per_transaction). While tempting to do something this simple, the most useful path to follow is probably to nail this head-on and comprehensively in the docs instead. Discussion of this topic on the hackers list seems to have concluded that connection pooling isn't as vital to do inside the database, as a high priority relative to other development, because it's addressed so well via external projects. Pointing people toward them seems quite appropriate given that position. Really addressing this well would take the following steps: -Add a section to the external projects section of the documentation: http://www.postgresql.org/docs/current/interactive/external-projects.html introducing connection pooling as a useful type of additional software to add. Shouldn't be controversial to suggest pgbouncer and pgpool-II as examples there. -Expand the documentation on max_connections to warn about how snapshot visibility overhead makes extremely large numbers of connections impractical -Also expand the documentation to suggest that CPU switching inefficiency may make a much smaller number of connections than expected optimal, and point toward the external project section for more information about pooling. -Add a warning to the postgresql.conf suggesting people read the documentation for max_connections before increasing this value. This area pops up enough that I've made a discussion of it part of even my shortest talk about PostgreSQL performance issues to be wary of. There's a good documentation patch project for somebody here, I just haven't had time to get to it yet. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Shared Buffer Size
On 05/27/2011 05:33 PM, preetika tyagi wrote: Hows does the shared buffer in Postgres rely on the Operating System cache? Suppose my RAM is 8 GB and shared_buffer is 24 MB in postgres. And there are some dirty pages in shared_buffer and I need to write a dirty page back to the disk to bring in a new page. What happens in this case? The dirty page will be written to the disk considering the shared_buffer size as 24 MB? or it will not be written and will stay in RAM which is 8 GB? In this case, the dirty page will be written to the operating system, which will initially store it in its own write cache. Eventually the operating system will write that page out to disk from that cache. The delay before that happens can easily be 30 seconds or more on operating systems like Linux. If PostgreSQL needs to read that data again, it may find it still in the OS cache, in which case the read will happen very quickly. Eventually, the dirty data will be written to disk, and if it's not used for long enough the OS cache memory will be re-used for something else. When you read a page into the database, and you don't dirty it, it might be evicted from the database cache without a write. If the database needs that page again, it will ask the OS for it. If the OS still has it in its own read cache, it may just read it from the cache again, without a real disk read happening. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Shared Buffer Size
On 05/28/2011 04:42 AM, Carl von Clausewitz wrote: I've try to get any information about optimal memory config, and working, but there wasn't any "optimal memory setting calculator" on the internet, just some guide in the posgre documentation There's a simple one at https://github.com/gregs1104/pgtune and the guide at http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server covers many of the most popular questions here too. The right proportion of memory setting is very dependent on workload, which makes any sort of calculator hard to create. pgtune takes a workload type as an input to help with that, but the settings that come out should be considered starting values only. You'll need to monitoring how much memory is actually being used by the server, as well as the output from parameters like log_time_files, to know for sure if things are working well. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgresql CBT
On 05/24/2011 02:05 AM, sade...@yahoo.com wrote: Id like to familiarize with postgresql and looking for a decent CBT but not able to find it. Most PostgreSQL training happens in one of the following ways: -Self-learning using the manual or one of the PostgreSQL books -In-person training at shared classrooms or on-site at companies who hire a trainer -Training offered as part of the many PostgreSQL conferences -Webcasts of training material You can find many of the schedules for these at http://www.postgresql.org/about/eventarchive , books are at http://www.postgresql.org/docs/books/ , and the large manual is at http://www.postgresql.org/docs/manuals/ The closest thing to CBT I know of are the videos recorded of past conference and user's group sessions. See http://vimeo.com/channels/postgres and http://fosslc.org/drupal/category/community/databases/postgresql for some samples. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgresql CBT
On 05/24/2011 10:49 AM, Scott Marlowe wrote: On Tue, May 24, 2011 at 8:36 AM, Vick Khera wrote: On Tue, May 24, 2011 at 10:33 AM, Scott Marlowe wrote: Id like to familiarize with postgresql and looking for a decent CBT but not able to find it. Could someone help pls? CBT? Please define. my guess is computer based training. Oh good. My first response from google, with safe search turned off, was much more distressing... : Cognitive behavioral therapy is only necessary for people migrating to PostgreSQL after using Access as if it were a database for too long. That's a very specific type of post-traumatic stress disorder, and mild cases can be treated with CBT. Severe cases will instead require ECT, aka electroshock. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: Fwd: Re: [GENERAL] SSDD reliability
On 05/04/2011 08:31 PM, David Boreham wrote: Here's my best theory at present : the failures ARE caused by cell wear-out, but the SSD firmware is buggy in so far as it fails to boot up and respond to host commands due to the wear-out state. So rather than the expected outcome (SSD responds but has read-only behavior), it appears to be (and is) dead. At least to my mind, this is a more plausible explanation for the reported failures vs. the alternative (SSD vendors are uniquely clueless at making basic electronics subassemblies), especially considering the difficulty in testing the firmware under all possible wear-out conditions. One question worth asking is : in the cases you were involved in, was manufacturer failure analysis performed (and if so what was the failure cause reported?). Unfortunately not. Many of the people I deal with, particularly the ones with budgets to be early SSD adopters, are not the sort to return things that have failed to the vendor. In some of these shops, if the data can't be securely erased first, it doesn't leave the place. The idea that some trivial fix at the hardware level might bring the drive back to life, data intact, is terrifying to many businesses when drives fail hard. Your bigger point, that this could just easily be software failures due to unexpected corner cases rather than hardware issues, is both a fair one to raise and even more scary. Intel claims their Annual Failure Rate (AFR) on their SSDs in IT deployments (not OEM ones) is 0.6%. Typical measured AFR rates for mechanical drives is around 2% during their first year, spiking to 5% afterwards. I suspect that Intel's numbers are actually much better than the other manufacturers here, so a SSD from anyone else can easily be less reliable than a regular hard drive still. Hmm, this is speculation I don't support (non-intel vendors have a 10x worse early failure rate). The entire industry uses very similar processes (often the same factories). One rogue vendor with a bad process...sure, but all of them ?? I was postulating that you only have to be 4X as bad as Intel to reach 2.4%, and then be worse than a mechanical drive for early failures. If you look at http://labs.google.com/papers/disk_failures.pdf you can see there's a 5:1 ratio in first-year AFR just between light and heavy usage on the drive. So a 4:1 ratio between best and worst manufacturer for SSD seemed possible. Plenty of us have seen particular drive models that were much more than 4X as bad as average ones among regular hard drives. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDD reliability
On 05/05/2011 10:35 AM, David Boreham wrote: On 5/5/2011 8:04 AM, Scott Ribe wrote: Actually, any of us who really tried could probably come up with a dozen examples--more if we've been around for a while. Original design cutting corners on power regulation; final manufacturers cutting corners on specs; component manufacturers cutting corners on specs or selling outright counterfeit parts... These are excellent examples of failure causes for electronics, but they are not counter-examples. They're unrelated to the discussion about SSD early lifetime hard failures. That's really optimistic. For all we know, these problems are the latest incarnation of something like the bulging capacitor plague circa 5 years ago. Some part that is unique to the SSDs other than the flash cells that there's a giant bad batch of. I think your faith in PC component manufacturing is out of touch with the actual field failure rates for this stuff, which is produced with enormous cost cutting pressure driving tolerances to the bleeding edge in many cases. The equipment of the 80's and 90's you were referring to ran slower, and was more expensive so better quality components could be justified. The quality trend at the board and component level has been trending for a long time toward cheap over good in almost every case nowadays. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: Fwd: Re: [GENERAL] SSDD reliability
On 05/04/2011 03:24 PM, David Boreham wrote: So if someone says that SSDs have "failed", I'll assume that they suffered from Flash cell wear-out unless there is compelling proof to the contrary. I've been involved in four recovery situations similar to the one described in that coding horror article, and zero of them were flash wear-out issues. The telling sign is that the device should fail to read-only mode if it wears out. That's not what I've seen happen though; what reports from the field are saying is that sudden, complete failures are the more likely event. The environment inside a PC of any sort, desktop or particularly portable, is not a predictable environment. Just because the drives should be less prone to heat and vibration issues doesn't mean individual components can't slide out of spec because of them. And hard drive manufacturers have a giant head start at working out reliability bugs in that area. You can't design that sort of issue out of a new product in advance; all you can do is analyze returns from the field, see what you screwed up, and do another design rev to address it. The idea that these new devices, which are extremely complicated and based on hardware that hasn't been manufactured in volume before, should be expected to have high reliability is an odd claim. I assume that any new electronics gadget has an extremely high failure rate during its first few years of volume production, particularly from a new manufacturer of that product. Intel claims their Annual Failure Rate (AFR) on their SSDs in IT deployments (not OEM ones) is 0.6%. Typical measured AFR rates for mechanical drives is around 2% during their first year, spiking to 5% afterwards. I suspect that Intel's numbers are actually much better than the other manufacturers here, so a SSD from anyone else can easily be less reliable than a regular hard drive still. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Question on Wal time lines
dabicho wrote: For restoring a database from wal files, if I omit a target on the recovery.conf file, can I make it so the database continues the time line instead of starting one? Or is there a tool to pick the most recent time line from a bunch of wal files? When recovery finishes, you get a new timeline. That way it's always possible to distinguish between a server that's exited recovery, and started generating new WAL data, from one that is still sync'd to the master and running recovery of the original timeline. If you don't want a new timeline, don't let recovery finish. As for the options you can tweak, see http://www.postgresql.org/docs/9.0/static/continuous-archiving.html#BACKUP-TIMELINES and the recovery target parameters at http://www.postgresql.org/docs/9.0/static/recovery-target-settings.html You can navigate among multiple timelines in a set of WAL files using recovery_target_timeline and the other target settings. It really is worth the trouble to run some experiments with these ideas to see what you can do, before you're forced to do so by an emergency. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Bidirectional replication
Merlin Moncure wrote: I know some people do some cool, usable things with that stuff, but the whole concept seems awfully awkward to me. I suppose I'm a crotchety, cane shaking fundamentalist... It's possible--do you sometimes find yourself yelling at young developers, telling them to stop replicating in your yard? -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
David Johnston wrote: Is there any rules-of-thumb on the performance of a PK as a function of key length? I like using varchar based identifiers since I tend to query tables directly and writing where clauses is much easier if you can avoid the joins. I'm likely better off creating views and querying those but am still curious on any basic thoughts on having a 100+ length primary key. The shorter the better, but it may not be as bad as you fear. The way B-tree indexes are built, it isn't that expensive to hold a longer key so long as the unique part doesn't average out to be that long. So if you insert "12345" and "12345777", that's not going to be much different than navigating "123456" and "123457", because once you get that far you've already reached a unique prefix. But if your entries have a really long common prefix, like "12" and "13", that's going to be more expensive to deal with--even though the strings are the same length. If your identifiers become unique after only a few characters, it may not be so bad. But if they go many characters before you can distinguish between any two entries, you're probably not going to be happy with the performance or size of the indexes, relative to simple integer keys. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
Jeff Davis wrote: On Mon, 2011-05-02 at 23:07 -0400, Greg Smith wrote: I see this whole area as being similar to SQL injection. The same way that you just can't trust data input by the user to ever be secure, you can't trust inputs to your database will ever be unique in the way you expect them to be. So, don't trust them to be unique then. Make up your own unique identifier, and use that. If you're making up your own unique identifier, that's closer to a surrogate key as far as I'm concerned, even though it doesn't fit the strict definition of that term (it doesn't have the subtle idea that "surrogate" implies "meaningless"). Now, there is some value to doing that well, instead of just using the typical incrementing integer "pointer" approach, as you've called it. But if it's not derived from external data you're storing anyway, it's not a true natural key either. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
Merlin Moncure wrote: If your data modeler that made the the assumptions that a MAC is unique (a mistake obviously) at least the other tables are protected from violations of that assumption because the database would reject them with an error, which is a Good Thing. Without a uniqueness constraint you now have ambiguous data which is a Very Bad Thing. With a uniqueness constraint in this situation, the unexpected data--row with a non unique MAC--will be rejected and possibly lost when the insertion happens. You say that's a good thing, plenty of people will say that's the worst possible thing that can happen. When dealing with external data, it's often impossible to know everything you're going to see later at design time. Approaching that problem with the idea that you're going to lose any data that doesn't fit into the original model is not what everyone finds reasonable behavior. I don't think it's possible to decide in a generic way which of these is the better approach: to reject unexpected data and force the problem back at the application immediately (commit failure), or to accept with with because you're using a surrogate key and discover the problems down the line. Both are valid approaches with a very different type of risk associated with them. I think it's fair to say that real-world data is not always well known enough at design time to follow the idea you're suggesting though, and that does factor into why there is such a preference for surrogate keys in the industry. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
John R Pierce wrote: otoh, there's plenty of places where natural keys are optimal. my company makes widgets, and we make damn sure our serial #s and part numbers are unique, and we use them as PK's for the various tables. Sure; what I was commenting on is that you normally can't ever trust external sources for identifiers. If you want to come up with your own, internally unique keys for things, great. But one of the goals of using a natural key is often to avoid the overhead of storing both that ID and some made up internal number, too. And whether the number is made up by the computer (the classic SERIAL or similar surrogate key), or you make one up yourself, it's still another chunk of data that gets stored for every item. It's just one that means something more useful in your case. Probably going to take up more space in the process and possibly be slower though--part number strings can easily end up longer than SERIAL-like integers. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
On 05/02/2011 10:06 PM, Rob Sargent wrote: You would be surprise how many "bob smith"s where born on the same day. But then they weren't all born in a hospital etc etc etc. I wouldn't be surprised. I once lived in a mile-square town (Hoboken, that's it's nickname). In that town were 40K residents and three gyms. I forgot my ID card one day when going to mine, and they took my name and street name as alternate proof of identity. Some designer along the line figured that was unique enough. Number of Greg Smiths living on that street who were members of that one gym? Three. I see this whole area as being similar to SQL injection. The same way that you just can't trust data input by the user to ever be secure, you can't trust inputs to your database will ever be unique in the way you expect them to be. So if you build a so-called "natural key" based on them, expect that to break one day. That doesn't mean you can't use them as a sort of foreign key indexing the data; it just means you can't make them the sole unique identifier for a particular entity, where that entity is a person, company, or part. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] pervasiveness of surrogate (also called synthetic) keys
On 05/01/2011 06:12 PM, Karsten Hilbert wrote: Good to know since I'm only a lowly medical doctor not having much schooling in database matters beyond this list, the PostgreSQL docs, and the Celko book. This debate exists at all levels of experience, and the only thing that changes as you get more experienced people involved is an increase in anecdotes on each side. The sole time I ever found myself arguing with Joe Celko is over an article he wrote recommending natural keys, using an example from the automotive industry. Problem was, the specific example he gave was flat out wrong. I was working in automotive MIS at the time, and the thing he was saying would never change did, in fact, change every year--in only a fraction of a percent of cases, in an extremely subtle way that snuck up on people and wreaked much confusion. That's typical for an early natural key design: you get it working fine in V1.0, only to discover months or years down the road there's a case you never considered you don't model correctly, and it may take some sort of conversion to fix. The reason why there's a strong preference for surrogate keys is that they always work and you can avoid ever needing to come up with a better design. if you just use them and forget about it. The position Merlin has advocated here, that there should always be a natural key available if you know the data well enough, may be true. But few people are good enough designers to be sure they've made the decision correctly, and the downsides of being wrong can be a long, painful conversion process. Easier for most people to just eliminate the possibility of making a mistake by using auto-generated surrogate keys, where the primary problem you'll run into is merely using more space/resources than you might otherwise need to have. It minimizes the worst-case--mistake make in the model, expensive re-design--by adding overhead that makes the average case more expensive. Software design usually has enough risks that any time you can eliminate one just by throwing some resources at it, that's normally the right thing to do. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Postgresql, PSN hack and table limits
On 05/01/2011 01:50 AM, Mark Morgan Lloyd wrote: Somebody is making a very specific claim that Postgres can support a limited number of rows Did you find this via http://www.reversecurity.com/2011/04/new-details-from-psn-hack.html ? That was the only Google-indexed source leading to it I found. I just left a note there about the silliness of these claims. I could run more than a 10M row PostgreSQL instance on my phone. Unless there's a new 16-bit only Vic 20 port of PostgreSQL available or something, it's seems unlikely the data had to be partitioned due to any hard limit. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Switching Database Engines
On 04/28/2011 12:19 PM, Carlos Mennens wrote: It seems that the 'mysql2postgres.pl' tool has instructions embedded into the file so I ran the command as instructed to take the output file and insert it into my PostgreSQL server and got the following error message: $ psql -p 5432 -h db1 -U wiki -f mediawiki_upgrade.pg Password for user wiki: BEGIN SET SET SET psql:mediawiki_upgrade.pg:25: ERROR: relation "category" does not exist My guess is that you need to run the main MediaWiki installer for PostgreSQL first, to create a blank install, in order for the category table to exist. The export tool is aimed to get the data out, not the schema to create all the tables. After you create a blank instance, then you do the data export. If you have additional problems, try running that like this instead: $ psql -p 5432 -h db1 -U wiki -e -f mediawiki_upgrade.pg Note the extra "-e" on the command line. That will show you the line it is executing as the script runs, so you'll see the one that fails too. Very handy for debugging what's gone wrong in this sort of situation. I wouldn't fight with this too much though. Unless you have some really customized stuff in your wiki, there really is nothing wrong with the idea of dumping everything into XML, creating a blank PostgreSQL-backed MediaWiki install, then restoring into that. That's what I always do in order to get a plain text backup of my server, and to migrate a wiki from one server to another. There are all kinds of issues you could have left here before this works, trying to do a database-level export/reload--encoding, foreign key problems, who knows what else. The database-agnostic export/import into XML avoids all of those. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Looking for Silicon Valley/Peninsula/San Francisco users group
On 04/29/2011 06:13 PM, Jeff Davis wrote: I'm not sure which reference you found, but SFPUG is certainly active with meetings every month. http://pugs.postgresql.org/sfpug ; last meeting listed there is January 2009. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDs with Postgresql?
On 04/29/2011 06:42 AM, Scott Marlowe wrote: I think you misunderstood. He's not storing 480GB on the drives, that's how much WAL is moving across it. It could easily be a single 80GB SSD drive or something like that. Right; that's why you don't necessarily get saved by the fact that larger databases must go onto more flash cells, too. Sometimes, yes, but not always. The WAL is really close to a worst-case for flash: lots of redundant information that's constantly overwritten. It's the last thing you want to consider putting onto SSD. There's a good reason why so many of the "enterprise" SSDs try to distinguish themselves with redundancy and wear leveling advancements; it's so this sort of workload doesn't kill them. Combine that workload possibility with the limitations of MLC flash, and you can see why the lifetimes actually are a serious concern in some situations. Not all of them, of course, but this is why I recommend things like directly measuring your WAL volume. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Partitioning an existing table
Vick Khera wrote: I've been trying to get the OSCON folk to accept this talk for several years now, to reach a wider audience. Seems they don't like me... :( Too specific for OSCON. It's not you, it's the topic. I've tried submitting far more generic things than that, but still with a PostgreSQL tilt to them, and I never get those accepted either. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDs with Postgresql?
On 04/26/2011 10:30 AM, Toby Corkindale wrote: I see Intel is/was claiming their SLC SSDs had a *minimum* lifetime of 2PB in writes for their 64GB disks; for your customer with a 50GB db and 20GB/day of WAL, that would work out at a minimum lifetime of a million days, or about 273 years! The cheaper "consumer grade" MLC drives should still last minimum 5 years at 20GB/day according to their literature. (And what I found was fairly out of date) That doesn't seem too bad to me - I don't think I've worked anywhere that keeps their traditional spinning disks in service beyond 5 years either. The comment I made there was that the 20GB/day system was a very small customer. One busy server, who are also the ones most likely to want SSD, I just watched recently chug through 16MB of WAL every 3 seconds=450GB/day. Now, you're right that those systems also aren't running with a tiny amount of flash, either. But the write volume scales along with the size, too. If you're heavily updating records in particular, the WAL volume can be huge relative to the drive space needed to store the result. As for the idea that I'm just singling out one anecdote, I have terabytes of lost data on multiple systems behind my negativity here. I was just pointing out a public failure that included some post-mortem I liked. I'm not sure if I have any happy customers who were early adopters of regular SLC or MLC drives really; the disaster rate is very close to 100% for the first few generations of those drives I've seen, and I've been around 50-ish of them. I'm hoping the current models shipping now are better, getting the write cache stuff sorted out better will be a big help. But it's been a scary technology for database use so far. The published numbers from the manufacturer literature are a very rosy best case when you're hitting the disk with this type of workload. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] PG 9.0 - Default postgresql.conf?
Phoenix Kiula wrote: What changes does initdb make -- can I make them by myself? The main thing is that it adjusts shared_buffers down until the server will start without exceeding the OS limits. It also sets some locale parameters within the database. Btw, the default file is FAILING. The server does not restart. How can I check what the problem is? I don't see any errors at least on the command line.All I see is: The errors are probably in /var/lib/pgsql/9.0/pgstartup.log Most of the time the problem is either: a) shared_buffers and the other memory parameters are set too high for the OS settings. b) There is already an instance running, or some file left behind from an earlier instance, that is blocking startup of the new server. c) A file is missing, corrupted, or has the wrong permissions All of the common cases identify themselves clearly in the log file. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Partitioning an existing table
On 04/25/2011 10:10 AM, Vick Khera wrote: Basically, you create your partitions and set up the necessary triggers you want (I re-write the app to insert directly into the correct partition). Then all new data starts going into the partitions. Next, write a program that loops over the current master table, and moves the data into each partition some small hunk at a time, in a transaction. This can take a long time. For us, it took about 7 days to move O(100m) rows. Then, when you're done, truncate the master table, and enforce that no new data is allowed to be inserted into it. Vick's presentation at http://cdn.mailermailer.com/documents/PGCon2008TablePartitioningCaseStudy.pdf is still one of the best case studies of how to do this sort of migration around. I was inspired by several of the ideas there when doing the partitioning chapter of my book, which is the only place I'm aware of covering this in even more detail than his case study. Cedric's idea for how to do this even more aggressively (multiple workers) is what you want if this is a one-time operation you're taking the system down for. In other situations, the gradual migration strategy Vick is suggesting is more appropriate. Some of the other ideas suggested in this thread won't work at all, so be careful who you listen to here. You can't leave copies of the data in the parent and put it into the child partition without all sorts of potential downsides. And you really, really want to do this as a proper database transaction, which is easiest to express using INSERT instead of COPY. If any step of the migration goes wrong, being able to do ROLLBACK and undo the recent bad steps is vital. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] 10 missing features
On 04/25/2011 04:54 PM, Nicholson, Brad (Toronto, ON, CA) wrote: The problem is that there is a lot of noise in the add-on space. There are lots of things out there that are no longer supported or partially supported. There is a fairly high barrier of entry into figuring out which tools to use, how to put them together and what you can and can't do with them. Right. This is why I advise people to start on it as soon as possible, to make it part of the initial testing of PostgreSQL. You have to start early on figuring which of the additional packages make sense for you, because in some environments you can't easily introduce them later if they weren't part of earlier QA. I tried to work on the barrier to entry part in my book, there's a lot of specific recommendations for add-ons in there and context about what they are and aren't good for. If (and I stress the word if) the target is winning over DBA's from the commercial crowd this is an important point, as those DBA's are going to be used to getting most of what they need in one package along with the DB. Unfortunately that whole line of thinking is fundamentally incompatible with how open source databases are built and packaged. What some people don't seem to get is that the "one package" here is "one operating system distribution with a good package manager". It's not like you have to build all this stuff from source or anything; in many cases the packages are available to add with a quick search and a few clicks. I do think the areas that are lacking in PG though do come to finer grain profiling of tasks. The ability to isolate CPU and IO utilization of particular queries or sets of queries is something I find very useful in the commercial DB space that I'd love to see in Postgres. Same goes for troubleshooting locking conflicts if you aren't looking at the system when they are happening, and tracing the causes of those locks down to finer grained details (IE - am I waiting on buffer eviction or xlog writes). This is all true. Unfortunately the way I expect this areas to be addressed doesn't start with "how can PostgreSQL duplicate the Oracle solution to this problem", which is how many of these incoming requests for features start. The alternate question of "how do you provide something with the same feature set for PostgreSQL?" is the more useful one, and it doesn't lead to the same solutions. That disconnect is important to address. If people are only willing to work toward or fund solving a familiar problem, they may not recognize an alternate solution that is just as useful--just not as familiar--that is available or being built. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] 10 missing features
On 04/25/2011 10:48 AM, Andrew Sullivan wrote: You can see this in certain items in the top 10. Three, four, five, seven, maybe 8 eight, and ten all seemed to me to be things I've actually done before, but not using something directly inside Postgres. The idea that something must ship in the database to be useful is really engrained in some people. I do this talk nowadays about common mistakes people make when deploying PostgreSQL, and one of the top items I put on there is not actively investigating external tools. None of the items on this list would be on my own top list of missing things in PostgreSQL. I see "Better fragmentation management" as a footnote and there's an intro discussion to that on the blog at http://blog.kimiensoftware.com/2011/04/compacting-postgresql-tables/ Apparently the struggles required to sort out a little 25GB table apparently didn't make enough of an impression to put that into its proper place, which is way ahead of every item listed on the suggested missing feature set. Query progress is #1? It's annoying, yes, but so not even close to pole position to me. From reading the blog a bit, it sounds like the author is managing lots of smallish (to me) databases, so putting so much emphasis on making each individual one easier to troubleshoot makes more sense. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Poor performance of btrfs with Postgresql
On 04/21/2011 02:22 AM, Toby Corkindale wrote: I also tested btrfs, and was disappointed to see it performed *dreadfully* - even with the recommended options for database loads. Best TPS I could get out of ext4 on the test machine was 2392 TPS, but btrfs gave me just 69! This is appalling performance. (And that was with nodatacow and noatime set) I don't run database performance tests until I've tested the performance of the system doing fsync calls, what I call its raw commit rate. That's how fast a single comitting process will be able to execute individual database INSERT statements for example. Whether or not barriers are turned on or not is the biggest impact on that, and from what you're describing it sounds like the main issue here is that you weren't able to get btrfs+nobarrier performing as expected. If you grab http://projects.2ndquadrant.it/sites/default/files/bottom-up-benchmarking.pdf page 26 will show you how to measure fsync rate directly using sysbench. Other slides cover how to get sysbench working right, you'll need to get a development snapshot to compile on your Ubuntu system. General fsync issues around btrfs are still plentiful it seems. Installing packages with dpkg sometimes does that (I haven't been following exactly which versions of Ubuntu do and don't fsync), so there are bug reports like https://bugs.launchpad.net/ubuntu/+source/dpkg/+bug/570805 and https://bugs.launchpad.net/ubuntu/+source/dpkg/+bug/607632 One interesting thing from there is an idea I'd never though of: you can link in an alternate system library that just ignore fsync if you want to test turning it off above the filesystem level. Someone has released a package to do just that, libeatmydata: http://www.flamingspork.com/projects/libeatmydata/ -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Poor performance of btrfs with Postgresql
On 04/21/2011 06:16 AM, Henry C. wrote: Since Pg is already "journalling", why bother duplicating (and pay the performance penalty, whatever that penalty may be) the effort for no real gain (except maybe a redundant sense of safety)? ie, use a non-journalling battle-tested fs like ext2. The first time your server is down and unreachable over the network after a crash, because it's run fsck to recover, failed to execute automatically, and now requires manual intervention before the system will finish booting, you'll never make that mistake again. On real database workloads, there's really minimal improvement to gain for that risk--and sometimes actually a drop in performance--using ext2 over a properly configured ext3. If you want to loosen the filesystem journal requirements on a PostgreSQL-only volume, use "data=writeback" on ext3. And I'd still expect ext4/XFS to beat any ext2/ext3 combination you can come up with, performance-wise. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDs with Postgresql?
On 04/21/2011 11:33 AM, Florian Weimer wrote: Is there an easy way to monitor WAL traffic in away? It does not have to be finegrained, but it might be helpful to know if we're doing 10 GB, 100 GB or 1 TB of WAL traffic on a particular database, should the question of SSDs ever come up. You can use functions like pg_current_xlog_location() : http://www.postgresql.org/docs/9.0/interactive/functions-admin.html Save a copy of this periodically: select now(),pg_current_xlog_location(); And you can see WAL volume over time given any two points from that set of samples. To convert the internal numbers returned by that into bytes, you'll need to do some math on them. Examples showing how that works and code in a few languages: http://archives.postgresql.org/pgsql-general/2010-10/msg00077.php (by hand) http://munin-monitoring.org/browser/trunk/plugins/node.d/postgres_streaming_.in?rev=3905 (in Perl) http://archives.postgresql.org/pgsql-general/2010-10/msg00079.php (in C) http://postgresql.1045698.n5.nabble.com/How-can-we-tell-how-far-behind-the-standby-is-td3252297.html (in bash with bc(!), other links) What I keep meaning to write is something that does that as part of the SQL itself, so it gets pulled out of the database already in bytes. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDs with Postgresql?
On 04/20/2011 01:50 AM, Toby Corkindale wrote: Also, the number of erase cycles you can get, over the whole disk, is quite large on modern disks! So large that you'll probably go decades before you wear the disk out, even with continual writes. Don't buy into the SSD FUD myths.. There is no FUD being spread here. Particularly given the PostgreSQL WAL write pattern, it's not impossible to wear out a SSD placed there in a small number of years. A system with a trivial but not completely idle workload will generate one 16MB WAL segment every 5 minutes, which works out to 4.5GB/day of writes. That's the baseline--the reality is much, much higher than that on most systems. The fact that every row update can temporarily use more than 8K means that actual write throughput on the WAL can be shockingly large. The smallest customer I work with regularly has a 50GB database, yet they write 20GB of WAL every day. You can imagine how much WAL is generated daily on systems with terabyte databases. As for what this translates into in the real world, go read http://archives.postgresql.org/message-id/BANLkTi=GsyBfq+ApWPR_qCA7AN+NqT=z...@mail.gmail.com as one worked out sample. Anyone deploying PostgreSQL onto MLC can't necessarily ignore this issue. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SSDs with Postgresql?
Henry C. wrote: I believe this perception that SSDs are less "safe" than failure-prone mechanical hard drives will eventually change. Only because the manufacturers are starting to care about write durability enough to include the right hardware for it. Many of them are less safe right now on some common database tasks. Intel's gen 1 and gen 2 drives are garbage for database use. I've had customers lose terabytes of data due to them. Yes, every system can fail, but these *will* fail and corrupt your database the first time there's a serious power problem of some sort. And the idea that a UPS is sufficient to protect against that even happening in wildly optimistic. See http://wiki.postgresql.org/wiki/Reliable_Writes for more background here, and links to reading on the older Intel drives. I summarized the situation with their newer 320 series drives at http://blog.2ndquadrant.com/en/2011/04/intel-ssd-now-off-the-sherr-sh.html Those finally get the write flushing right. But the random seeks IOPS is wildly lower than you might expect on read/write workloads. My own tests and other sources have all come up with around 3500 IOPS as being a real-world expectation for the larger sizes of these drives. Also, it is cheap flash, so durability in a server environment won't be great. Don't put your WAL on them if you have a high transaction rate. Put some indexes there instead. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Raid Controller Write Cache setting for WAL and Data
Piotr Gasidło wrote: I _can_ afford of loosing some data in case of power failure. But I'm afraid of having database in unrecoverable state after crash. Then turn off synchronous_commit. That's exactly the behavior you get when it's disabled: some data loss after a crash, no risk of database corruption, and faster performance without needing a controller with a battery. If you've already got a RAID controller that accepts a battery, it would be silly not to then buy one though. The controller is normally 75% of the price of the combination, so getting that but not the final piece to really make it perform well wouldn't be a good move. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] question regarding full_page_write
AI Rumman wrote: I can't clearly understand what FULL_PAGE_WRITE parameter is stand for. Documentation suggest that If I make it OFF, then I have the chance for DB crash. Can anyone please tell me how it could be happened? The database writes to disk in 8K blocks. If you can be sure that your disk drives and operating system will always write in 8K blocks, you can get a performance improvement from turning full_page_writes off. But if you do that, and it turns out that when the power is interrupted your disk setup will actually do partial writes of less than 8K, your database can get corrupted. Your system needs to ensure that when a write happens, either the whole thing goes to disk, or none of it does. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] effective_io_concurrency
Yves Weißig wrote: I was wondering if there is more information about this switch in the configuration. Does it really work? Where in the source code can I follow how it works? "sgmgr.c" seems to be an entry point, but where exactly is it used? Currently the code only kicks in when you're doing a Bitmap Heap Scan, which is really helpful for them, but of no help for any other type of query. This style of heap scan already knows in advance exactly what blocks it needs from the database, and normally it just asks for them one at a time. That can turn into a fair amount of random I/O, and it's done serially: the next block isn't requested until the last one arrives. What effective_io_concurrency does is advise the operating system of the next few blocks the database is going to ask for, before the actual read requests, in hopes that it might grab them if it happens to be nearby that area of the disk.e I've only seen this feature work at all on Linux. It might work on BSD and Mac OS X systems, it certainly doesn't do anything on Solaris and Windows. The basic idea is that you start with setting the value to the number of working drives in the disk array the database is on and see if I/O performance goes up and/or query speed drops afterwards. If it does you might try further increases beyond that even. As for why there isn't a better tuning guide than just those simple guidelines, it's not that easy to show a situation where the type of bitmap scan this parameter impacts is used on a generated data set, even though it's not that uncommon in real-world data. It's hard both to make generic suggestions here and to even demonstrate the feature at work. Moving up the source code chain from smgr, src/backend/storage/buffer/bufmgr.c has PrefetchBuffer, and the one place the executor calls that is BitmapHeapNext inside src/backend/executor/nodeBitmapHeapscan.c -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Book recommendation?
Herouth Maoz wrote: My sysadmin ran into the following book: PostgreSQL 9.0 High Performance, by Gregory Smith, ISBN 184951030X http://amzn.com/184951030X That guy's a troublemaker, but I guess he writes OK. There are three customer reviews at http://www.amazon.com/PostgreSQL-High-Performance-Gregory-Smith/product-reviews/184951030X/ and two longer ones at: http://people.planetpostgresql.org/andrew/index.php?/archives/130-Buy-this-book,-now..html http://www.postgresonline.com/journal/archives/192-postgresql9highperformance.html And here's some free samples: https://www.packtpub.com/article/postgresql-9-reliable-controller-disk-setup https://www.packtpub.com/article/postgresql-9-balancing-hardware-spending https://www.packtpub.com/article/server-configuration-tuning-postgresql https://www.packtpub.com/article/unix-monitoring-tool-for-postgresql https://www.packtpub.com/article/postgresql-tips-tricks https://www.packtpub.com/sites/default/files/0301OS-Chapter-2-Database-Hardware.pdf -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] SHMMAX and SHMALL question
DM wrote: RAM = 16GB, what value should i set for shmall? Given that PostgreSQL rarely sees increasing improvement as shared_buffers goes over 50% of RAM, I just use that figure for the shmall and then compute shmmax based on the page size to match it. I use the attached script to do all the hard work, haven't found a Linux system yet it didn't do the right thing on. It sounds like you might have the math on the relation between the two backwards, look at the output and code of this once and that should sort things out for you. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books #!/bin/bash # Output lines suitable for sysctl configuration based # on total amount of RAM on the system. The output # will allow up to 50% of physical memory to be allocated # into shared memory. # On Linux, you can use it as follows (as root): # # ./shmsetup >> /etc/sysctl.conf # sysctl -p # Early FreeBSD versions do not support the sysconf interface # used here. The exact version where this works hasn't # been confirmed yet. page_size=`getconf PAGE_SIZE` phys_pages=`getconf _PHYS_PAGES` if [ -z "$page_size" ]; then echo Error: cannot determine page size exit 1 fi if [ -z "$phys_pages" ]; then echo Error: cannot determine number of memory pages exit 2 fi shmall=`expr $phys_pages / 2` shmmax=`expr $shmall \* $page_size` echo \# Maximum shared segment size in bytes echo kernel.shmmax = $shmmax echo \# Maximum number of shared memory segments in pages echo kernel.shmall = $shmall -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] migrate hashname function from 8.1.x to 8.4
Nicolas Garfinkiel wrote: I'm trying to upgrade our pgsql from 8.1 to 8.4, but our system's login uses the hashname() function in order to get the proper password validation. Now pgsql's 8.4 hashname function is not compatible with 8.1's function. Do you have any ideas how I can reproduce 8.1 function in 8.4? https://github.com/petere/pgvihash provides the function you're looking for. I agree with Craig's concerns here, but this may let you convert toward a better long-term format more easily. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
Re: [GENERAL] PG84 and SELinux
Tom Lane wrote: "James B. Byrne" writes: I wrote too soon. What I did was uncomment the ssl option. I neglected to change the setting from off to on. When I try to start the server with ssl=on it fails with this error: Auto configuration failed 29006:error:0E065068:configuration file routines:STR_COPY:variable has no value:conf_def.c:629:line 207 AFAIK there is no place in the standard Postgres sources that could emit an error message even vaguely like that. That looks to be the str_copy routine from conf_def.c in the OpenSSL code, i.e. line 624 of the version at: http://code.google.com/p/commitmonitor/source/browse/trunk/common/openssl/crypto/conf/conf_def.c So guessing something in the SSL autonegotiation is failing here in a really unexpected way. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services and Supportwww.2ndQuadrant.us
Re: [GENERAL] Considering Solid State Drives
Allan Kamau wrote: I am now thinking of investing in a SSD (Solid State Drive), and maybe choosing between "Crucial Technology 256GB Crucial M225 Series 2.5-Inch Solid State Drive (CT256M225)" and "Intel X25-M G2 (160GB) - Intel MLC". Both of these are worthless for database applications if you care about your data. In order to perform well, SSDs need to have a write cache to buffer small writes. For PostgreSQL to work as intended, that write cache needs to be non-volatile. It is not in either of those drives. I hear tales of lost PostgreSQL data on Intel SSDs every month, the database is lucky to survive a single power outage. The only relatively inexpensive SSD we've heard about on the Performance list that's survived all of the durability on crash tests thrown at it is the OCZ Vertex 2 Pro; see http://archives.postgresql.org/pgsql-performance/2010-07/msg00449.php for a summary. That avoids this problem by having an Ultracapacitor integrated with the drive, to allow orderly processing of the write cache if power is lost. There are other SSD devices that are similarly reliable, but the costs are quite a bit higher. More background about this topic at http://wiki.postgresql.org/wiki/Reliable_Writes -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services and Supportwww.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] The first dedicated PostgreSQL forum
Tom Lane wrote: I'm not nearly as concerned about whether there are forums as about having "rogue" forums outside the postgresql.org domain. People could misperceive such things as having some official status That the site is now mirroring recent news from postgresql.org doesn't help with that. I find it hard to get too excited about yet another forum style discussion area when there's already more PostgreSQL talk on http://stackoverflow.com/ than I have time to keep up with. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services and Supportwww.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
