From: Andres Freund <andres@anarazel.de>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Scott Carey <scott@richrelevance.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Kenneth Cox <kenstir@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Advice configuring ServeRAID 8k for performance
Date: Mon, 16 Aug 2010 22:02:27 +0200
Message-ID: <20100816200227.GA3911@anarazel.de> (raw)
In-Reply-To: <4C69796D.9000408@2ndquadrant.com>
References: <201008131817.o7DIH1b03550@momjian.us>
<4C6591D9.1080206@2ndquadrant.com>
<5687A8AD-CA70-4272-A32C-6AE5F491CEA4@richrelevance.com>
<4C69796D.9000408@2ndquadrant.com>
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
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To: pgsql-performance@postgresql.org
Cc: andres@anarazel.de, greg@2ndquadrant.com, scott@richrelevance.com, bruce@momjian.us, kenstir@gmail.com
Subject: Re: Advice configuring ServeRAID 8k for performance
In-Reply-To: <20100816200227.GA3911@anarazel.de>
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