Message-ID: <4CD737C1.9070502@2ndquadrant.com>
Date: Sun, 07 Nov 2010 18:35:29 -0500
From: Greg Smith <greg@2ndquadrant.com>
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To: Marti Raudsepp <marti@juffo.org>
CC: Andres Freund <andres@anarazel.de>, pgsql-performance@postgresql.org
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for
 9.1?
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Marti Raudsepp wrote:
> I will grant you that the details were wrong, but I stand by the conclusion.
> I can state for a fact that PostgreSQL's default wal_sync_method
> varies depending on the <fcntl.h> header.
>   

Yes; it's supposed to, and that logic works fine on some other 
platforms.  The question is exactly what the new Linux O_DSYNC behavior 
is doing, in regards to whether it flushes drive caches out or not.  
Until you've quantified which of the cases do that--which is required 
for reliable operation of PostgreSQL--and which don't, you don't have 
any data that can be used to draw a conclusion from.  If some setups are 
faster because they write less reliably, that doesn't automatically make 
them the better choice.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books