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Re: PostgreSQL as a local in-memory cache

From: Jignesh Shah <jkshah(at)gmail(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
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 00:48:23
Message-ID: AANLkTikj3pglP8uqPdUBHZ3bDQq86aG6MYzDUUyurTez@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
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..

Regards,
Jignesh

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