Re: [PERFORM] Is DBLINK transactional
On 13/03/2010 5:54 AM, Jeff Davis wrote: On Fri, 2010-03-12 at 12:07 -0500, Merlin Moncure wrote: of course. You can always explicitly open a transaction on the remote side over dblink, do work, and commit it at the last possible moment. Your transactions aren't perfectly synchronized...if you crash in the precise moment between committing the remote and the local you can get in trouble. The chances of this are extremely remote though. If you want a better guarantee than that, consider using 2PC. Translation in case you don't know: 2PC = two phase commit. Note that you have to monitor lost transactions that were prepared for commit then abandoned by the controlling app and periodically get rid of them or you'll start having issues. The problem with things that are extremely remote possibilities are that they tend to be less remote than we expect ;) ... and they know just when they can happen despite all the odds to maximise the pain and chaos caused. -- Craig Ringer -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Deleting bytea, autovacuum, and 8.2/8.4 differences
Hi all, my posting on 2010-01-14 about the performance when writing bytea to disk caused a longer discussion. While the fact still holds that the overall postgresql write performance is roughly 25% of the serial I/O disk performance this was compensated for my special use case here by doing some other non-postgresql related things in parallel. Now I cannot optimize my processes any further, however, now I am facing another quite unexpected performance issue: Deleting rows from my simple table (with the bytea column) having 16 MB data each, takes roughly as long as writing them! Little more detail: * The table just has 5 unused int columns, a timestamp, OIDs, and the bytea column, no indices; the bytea storage type is 'extended', the 16 MB are compressed to approx. the half. * All the usual optimizations are done to reach better write through (pg_xlog on another disk, much tweaks to the server conf etc), however, this does not matter here, since not the absolute performance is of interest here but the fact that deleting roughly takes 100% of the writing time. * I need to write 15 rows of 16 MB each to disk in a maximum time of 15 s, which is performed here in roughly 10 seconds, however, now I am facing the problem that keeping my database tidy (deleting rows) takes another 5-15 s (10s on average), so my process exceeds the maximum time of 15s for about 5s. * Right now I am deleting like this: DELETE FROM table WHERE (CURRENT_TIMESTAMP - my_timestamp_column) interval '2 minutes'; while it is planned to have the interval set to 6 hours in the final version (thus creating a FIFO buffer for the latest 6 hours of inserted data; so the FIFO will keep approx. 10.000 rows spanning 160-200 GB data). * This deletion SQL command was simply repeatedly executed by pgAdmin while my app kept adding the 16 MB rows. * Autovacuum is on; I believe I need to keep it on, otherwise I do not free the disk space, right? If I switch it off, the deletion time reduces from the average 10s down to 4s. * I am using server + libpq version 8.2.4, currently on WinXP. Will an upgrade to 8.4 help here? Do you have any other ideas to help me out? Oh, please... Thank You Felix -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Deleting bytea, autovacuum, and 8.2/8.4 differences
Hi there I'm not an expert on PG's toast system, but a couple of thoughts inline below. Cheers Dave On Sat, Mar 13, 2010 at 3:17 PM, fka...@googlemail.com fka...@googlemail.com wrote: Hi all, my posting on 2010-01-14 about the performance when writing bytea to disk caused a longer discussion. While the fact still holds that the overall postgresql write performance is roughly 25% of the serial I/O disk performance this was compensated for my special use case here by doing some other non-postgresql related things in parallel. Now I cannot optimize my processes any further, however, now I am facing another quite unexpected performance issue: Deleting rows from my simple table (with the bytea column) having 16 MB data each, takes roughly as long as writing them! Little more detail: * The table just has 5 unused int columns, a timestamp, OIDs, and the bytea column, *no indices*; the bytea storage type is 'extended', the 16 MB are compressed to approx. the half. Why no indices? * All the usual optimizations are done to reach better write through (pg_xlog on another disk, much tweaks to the server conf etc), however, this does not matter here, since not the absolute performance is of interest here but the fact that deleting roughly takes 100% of the writing time. * I need to write 15 rows of 16 MB each to disk in a maximum time of 15 s, which is performed here in roughly 10 seconds, however, now I am facing the problem that keeping my database tidy (deleting rows) takes another 5-15 s (10s on average), so my process exceeds the maximum time of 15s for about 5s. * Right now I am deleting like this: DELETE FROM table WHERE (CURRENT_TIMESTAMP - my_timestamp_column) interval '2 minutes'; You *need* an index on my_timestamp_column while it is planned to have the interval set to 6 hours in the final version (thus creating a FIFO buffer for the latest 6 hours of inserted data; so the FIFO will keep approx. 10.000 rows spanning 160-200 GB data). That's not the way to keep a 6 hour rolling buffer ... what you need to do is run the delete frequently, with *interval '6 hours'* in the SQL acting as the cutoff. If you really do want to drop the entire table contents before refilling it, do a *DROP TABLE* and recreate it. * This deletion SQL command was simply repeatedly executed by pgAdmin while my app kept adding the 16 MB rows. Are you sure you are timing the delete, and not pgAdmin re-populating some kind of buffer? * Autovacuum is on; I believe I need to keep it on, otherwise I do not free the disk space, right? If I switch it off, the deletion time reduces from the average 10s down to 4s. You may be running autovaccum too aggressively, it may be interfering with I/O to the tables. Postgres vacuuming does not free disk space (in the sense of returning it to the OS), it removes old versions of rows that have been UPDATEd or DELETEd and makes that space in the table file available for new writes. * I am using server + libpq version 8.2.4, currently on WinXP. Will an upgrade to 8.4 help here? 8.4 has a lot of performance improvements. It's definitely worth a shot. I'd also consider switching to another OS where you can use a 64-bit version of PG and a much bigger buffer cache. Do you have any other ideas to help me out? Oh, please... Thank You Felix -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
