| 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: | 4B7646D2.3010508@2ndquadrant.com |
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| Thread: | |
| 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
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com
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