To: Bruce Momjian <bruce@momjian.us>
cc: Robert Haas <robertmhaas@gmail.com>,
	Dimitri Fontaine <dfontaine@hi-media.com>, Josh Berkus <josh@agliodbs.com>,
	postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: PostgreSQL as a local in-memory cache 
In-reply-to: <201006291719.o5THJeK29759@momjian.us> 
References: <201006291719.o5THJeK29759@momjian.us>
Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
	message dated "Tue, 29 Jun 2010 13:19:40 -0400"
Date: Tue, 29 Jun 2010 13:27:34 -0400
Message-ID: <20972.1277832454@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Bruce Momjian <bruce@momjian.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.

			regards, tom lane