On Jan 18, 2011, at 5:21 PM, Bruce Momjian wrote: > A.M. wrote: >> >> On Jan 18, 2011, at 5:16 PM, Bruce Momjian wrote: >> >>> A.M. wrote: >>>> >>>> On Jan 18, 2011, at 3:55 PM, Bruce Momjian wrote: >>>> >>>>> I have modified test_fsync to use test labels that match wal_sync_method >>>>> values, and and added more tests for open_sync with different sizes. >>>>> This should make the program easier for novices to understand. Here is >>>>> a test run for Ubuntu 11.04: >>>>> >>>>> $ ./test_fsync >>>>> 2000 operations per test >>>>> >>>>> Compare file sync methods using one 8k write: >>>>> (in wal_sync_method preference order, except fdatasync >>>>> is Linux's default) >>>>> open_datasync (non-direct I/O)* 85.127 ops/sec >>>>> open_datasync (direct I/O) 87.119 ops/sec >>>>> fdatasync 81.006 ops/sec >>>>> fsync 82.621 ops/sec >>>>> fsync_writethrough n/a >>>>> open_sync (non-direct I/O)* 84.412 ops/sec >>>>> open_sync (direct I/O) 91.006 ops/sec >>>>> * This non-direct I/O mode is not used by Postgres. >>>> >>>> I am curious how this is targeted at novices. A naive user might enable >>>> the "fastest" option which could be exactly wrong. For this to be useful >>>> to novices, I suspect the tool will need to generate platform-specific >>>> suggestions, no? >>> >>> Uh, why isn't the fastest option right for them? It is hardware/kernel >>> specific when you run it --- how could it be better? >> >> Because the fastest option may not be syncing to disk. For example, >> the only option that makes sense on OS X is fsync_writethrough- it >> would be helpful if the tool pointed that out (on OS X only, obviously). > > Yes, that would be a serious problem. :-( > > I am not sure how we would address this --- your point is a good one.
One general idea I had would be to offer some heuristics such as "this sync rate is comparable to that of one SATA drive" or "comparable to RAID 10 with X drives" or "this rate is likely too fast to be actually be syncing". But then you are stuck with making sure that the heuristics are kept up-to-date, which would be annoying. Otherwise, the only option I see is to detect the kernel and compare against a list of known problematic methods. Perhaps it would be easier to compare against a whitelist. Also, the tool would likely need to parse "mount" output to account for problems with specific filesystems. I am just throwing around some ideas... Cheers, M -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
