Subject: Re: Choosing a filesystem
From: Simon Riggs <simon@2ndQuadrant.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Merlin Moncure <mmoncure@gmail.com>, david@lang.hm,
	Greg Smith <gsmith@gregsmith.com>, Guillaume Cottenceau <gc@mnc.ch>,
	pgsql-performance@postgresql.org
In-Reply-To: <200809231702.m8NH21118074@momjian.us>
References: <200809231702.m8NH21118074@momjian.us>
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Date: Tue, 23 Sep 2008 20:01:15 +0100
Message-Id: <1222196475.4445.444.camel@ebony.2ndQuadrant>
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On Tue, 2008-09-23 at 13:02 -0400, Bruce Momjian wrote:
> Merlin Moncure wrote:
> > > although for postgres the thing that you are doing the fsync on is the WAL
> > > log file. that is a single (usually) contiguous file. As such it is very
> > > efficiant to write large chunks of it. so while you will degrade from the
> > > battery-only mode, the fact that the controller can flush many requests
> > > worth of writes out to the WAL log at once while you fill the cache with
> > > them one at a time is still a significant win.
> > 
> > The heap files have to be synced as well during checkpoints, etc.
> 
> True, but as of 8.3 those checkpoint fsyncs are spread over the interval
> between checkpoints.

No, the fsyncs still all happen in a tight window after we have issued
the writes. There's no waits in between them at all. The delays we
introduced are all in the write phase. Whether that is important or not
depends upon OS parameter settings.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support