Date: Thu, 7 Dec 2006 11:42:39 -0500
From: Bill Moran <wmoran@collaborativefusion.com>
To: pgsql-performance@postgresql.org
Subject: How to determine if my setting for shared_buffers is too high?
Message-Id: <20061207114239.c6b72e63.wmoran@collaborativefusion.com>
Organization: Collaborative Fusion
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I'm gearing up to do some serious investigation into performance for
PostgreSQL with regard to our application.  I have two issues that I've
questions about, and I'll address them in two seperate emails.

This one regards tuning shared_buffers.

I believe I have a good way to monitor database activity and tell when
a database grows large enough that it would benefit from more
shared_buffers: if I monitor the blks_read column of pg_stat_database,
it should increase very slowly if there is enough shared_buffer
space.  When shared buffer space runs out, more disk read requests
will be required and this number will begin to climb.

If anyone sees a flaw in this approach, I'd be interested to hear it.

The other tuning issue with shared_buffers is how to tell if I'm
allocating too much.  For example, if I allocate 1G of RAM to
shared buffers, and the entire database can fit in 100M, that 900M
might be better used as work_mem, or something else.

I haven't been able to find anything regarding how much of the
shared buffer space PostgreSQL is actually using, as opposed to
simply allocating.

-- 
Bill Moran
Collaborative Fusion Inc.

wmoran@collaborativefusion.com
Phone: 412-422-3463x4023