Message-ID: <4B7646D2.3010508@2ndquadrant.com>
Date: Sat, 13 Feb 2010 01:29:38 -0500
From: Greg Smith <greg@2ndquadrant.com>
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To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
CC: hannu@krosing.net, alvherre@commandprompt.com, aidan@highrise.ca,
	jesper@krogh.cc, pgsql-performance@postgresql.org
Subject: Re: moving pg_xlog -- yeah, it's worth it!
References: <4B750193020000250002F22D@gw.wicourts.gov>
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Kevin Grittner wrote:
> I wonder if it might also pay to make the background writer even more
> aggressive than we have, so that SELECT-only queries don't spend so
> much time writing pages.
You can easily quantify if the BGW is aggressive enough.  Buffers leave 
the cache three ways, and they each show up as separate counts in 
pg_stat_bgwriter:  buffers_checkpoint, buffers_clean (the BGW), and 
buffers_backend (the queries).  Cranking it up further tends to shift 
writes out of buffers_backend, which are the ones you want to avoid, 
toward buffers_clean instead.  If buffers_backend is already low on a 
percentage basis compared to the other two, there's little benefit in 
trying to make the BGW do more.

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