On Tue, 2010-06-29 at 21:39 -0400, Bruce Momjian wrote: > Jignesh Shah wrote: > > On Tue, Jun 29, 2010 at 2:45 PM, Bruce Momjian <bruce@xxxxxxxxxx> wrote: > > > Tom Lane wrote: > > >> Bruce Momjian <bruce@xxxxxxxxxx> writes: > > >> >>> I asked on IRC and was told it is true, and looking at the C code it > > >> >>> looks true. ?What synchronous_commit = false does is to delay writing > > >> >>> the wal buffers to disk and fsyncing them, not just fsync, which is > > >> >>> where the commit loss due to db process crash comes from. > > >> > > >> >> Ah, I see. ?Thanks. > > >> > > >> > I am personally surprised it was designed that way; ?I thought we would > > >> > just delay fsync. > > >> > > >> That would require writing and syncing to be separable actions. ?If > > >> you're using O_SYNC or similar, they aren't. > > > > > > Ah, very good point. ?I have added a C comment to clarify why this is > > > the current behavior; ?attached and applied. > > > > > > -- > > > ?Bruce Momjian ?<bruce@xxxxxxxxxx> ? ? ? ?http://momjian.us > > > ?EnterpriseDB ? ? ? ? ? ? ? ? ? ? ? ? ? ? http://enterprisedb.com > > > > > > Though has anybody seen a behaviour where synchronous_commit=off is > > slower than synchronous_commit=on ? Again there are two cases here > > one with O_* flag and other with f*sync flags. But I had seen that > > behavior with PostgreSQL 9.0 beta(2 I think) though havent really > > investigated it much yet .. (though now I dont remember which > > wal_sync_method flag) . Just curious if anybody has seen that > > behavior.. > > I have trouble believing how synchronous_commit=off could be slower than > 'on'. > I wonder if it could be contention on wal buffers? Say I've turned synchronous_commit off, I drive enough traffic fill up my wal_buffers. I assume that we would have to start writing buffers down to disk before allocating to the new process. -- Brad Nicholson 416-673-4106 Database Administrator, Afilias Canada Corp. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
