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From: Andres Freund <andres@anarazel.de>
To: pgsql-performance@postgresql.org
Subject: Re: Rather large LA
Date: Tue, 6 Sep 2011 16:51:17 +0200
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Cc: Scott Marlowe <scott.marlowe@gmail.com>,
	Richard Shaw <richard@aggress.net>, Andy Colson <andy@squeakycode.net>
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On Monday 05 Sep 2011 22:23:32 Scott Marlowe wrote:
> On Mon, Sep 5, 2011 at 11:24 AM, Andres Freund <andres@anarazel.de> wrote:
> > On Monday, September 05, 2011 14:57:43 Richard Shaw wrote:
> >> Autovacuum has been disabled and set to run manually via cron during a
> >> quiet period and fsync has recently been turned off to gauge any real
> >> world performance increase, there is battery backup on the raid card
> >> providing some level of resilience.
> > 
> > That doesn't help you against a failure due to fsync() off as the BBU can
> > only protect data that actually has been written to disk. Without
> > fsync=on no guarantee about that exists.
> 
> Further, if you've got a bbu cache on the RAID card the gains from
> fsync=off wll be low / nonexistent.
Thats not necessarily true. If you have a mixed load of many small writes and 
some parallel huge writes (especially in combination with big indexes) 
fsync=off still can give you quite big performance increases. Even in the 
presenence of synchronous_commit=off.

Andres