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Re: moving pg_xlog -- yeah, it's worth it!

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: hannu(at)krosing(dot)net, alvherre(at)commandprompt(dot)com, aidan(at)highrise(dot)ca, jesper(at)krogh(dot)cc, pgsql-performance(at)postgresql(dot)org
Subject: Re: moving pg_xlog -- yeah, it's worth it!
Date: 2010-02-13 06:29:38
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
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

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