Date: Mon, 16 Aug 2010 22:02:27 +0200
From: Andres Freund <andres@anarazel.de>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Scott Carey <scott@richrelevance.com>, Bruce Momjian <bruce@momjian.us>,
	Kenneth Cox <kenstir@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Advice configuring ServeRAID 8k for performance
Message-ID: <20100816200227.GA3911@anarazel.de>
References: <201008131817.o7DIH1b03550@momjian.us>
	<4C6591D9.1080206@2ndquadrant.com>
	<5687A8AD-CA70-4272-A32C-6AE5F491CEA4@richrelevance.com>
	<4C69796D.9000408@2ndquadrant.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4C69796D.9000408@2ndquadrant.com>
User-Agent: Mutt/1.5.20 (2009-06-14)

On Mon, Aug 16, 2010 at 01:46:21PM -0400, Greg Smith wrote:
> Scott Carey wrote:
> >This is because an fsync on ext3 flushes _all dirty pages in the file system_ to disk, not just those for the file being fsync'd.
> >One partition for WAL, one for data.  If using ext3 this is
> >essentially a performance requirement no matter how your array is
> >set up underneath.
>
> Unless you want the opposite of course.  Some systems split out the
> WAL onto a second disk, only to discover checkpoint I/O spikes
> become a problem all of the sudden after that.  The fsync calls for
> the WAL writes keep the write cache for the data writes from ever
> getting too big.  This slows things down on average, but makes the
> worst case less stressful.  Free lunches are so hard to find
> nowadays...
Or use -o sync. Or configure a ridiciuosly low dirty_memory amount
(which has a problem on large systems because 1% can still be too
much. Argh.)...

Andres