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Re: PostgreSQL as a local in-memory cache

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Jignesh Shah <jkshah(at)gmail(dot)com>
Cc: 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 01:39:17
Message-ID: 201006300139.o5U1dHN12285@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performance
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'.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + None of us is going to be here forever. +

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