Message-ID: <4C1B0C11.1010204@2ndquadrant.com>
Date: Fri, 18 Jun 2010 02:02:57 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Mark Mielke <mark@mark.mielke.cc>
CC: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>,
	pgsql-performance@postgresql.org
Subject: Re: wal_synch_method = open_sync safe on RHEL 5.5?
References: <4C1ACE5F.1080802@catalyst.net.nz>
	<4C1AE820.6020506@2ndquadrant.com>
	<4C1AE9A3.4080307@mark.mielke.cc>
In-Reply-To: <4C1AE9A3.4080307@mark.mielke.cc>
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Mark Mielke wrote:
> The conclusion I read was that Linux O_SYNC behaves like O_DSYNC on 
> other systems. For WAL, this seems satisfactory?

It would be if it didn't have any bugs or limitiations, but it does.  
The one pointed out in the message I linked to suggests that a mix of 
buffered and O_SYNC direct I/O can cause a write error, with the exact 
behavior you get depending on the kernel version.  That's a path better 
not explored as I see it.

The kernels that have made some effort to implement this correctly 
actually expose O_DSYNC, on newer Linux systems.  My current opinion is 
that if you only have Linux O_SYNC, don't use it.  The ones with O_DSYNC 
haven't been around for long enough to be proven or disproven as 
effective yet.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us