From: Andres Freund <andres@anarazel.de>
To: pgsql-performance@postgresql.org
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?
Date: Mon, 1 Nov 2010 00:10:28 +0100
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Cc: Greg Smith <greg@2ndquadrant.com>,
 Marti Raudsepp <marti@juffo.org>
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On Sunday 31 October 2010 20:59:31 Greg Smith wrote:
> Writes only are sync'd out when you do a commit, or the database does a 
> checkpoint.
Hm?  WAL is written out to disk after an the space provided by wal_buffers(def 
8) * XLOG_BLCKSZ (def 8192) is used. The default is 64kb which you reach 
pretty quickly - especially after a checkpoint. With O_D?SYNC that will 
synchronously get written out during a normal XLogInsert if hits a page 
boundary.
*Additionally* its gets written out at a commit if sync commit is not on.

Not having a real O_DSYNC on linux until recently makes it even more dubious 
to have it as a default...


Andres