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Re: PostgreSQL as a local in-memory cache

From: Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Jignesh Shah <jkshah(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: PostgreSQL as a local in-memory cache
Date: 2010-06-30 14:30:57
Message-ID: 1277908257.19899.67.camel@bnicholson-desktop (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)momjian(dot)us> wrote:
> > > Tom Lane wrote:
> > >> Bruce Momjian <bruce(at)momjian(dot)us> 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(at)momjian(dot)us> ? ? ? ?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.



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Subject: Re: PostgreSQL as a local in-memory cache
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