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7.4 Checkpoint Question

From: Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>
To: pgsql-performance(at)postgresql(dot)org
Subject: 7.4 Checkpoint Question
Date: 2007-11-29 15:10:54
Message-ID: 1196349054.5651.19.camel@bnicholson-desktop (view raw or flat)
Thread:
Lists: pgsql-performance
I have a legacy system still on 7.4 (I know, I know...the upgrade is
coming soon).

I have a fairly big spike happening once a day, every day, at the same
time.  It happens during a checkpoint, no surprise there.  I know the
solution to the problem (upgrade to a modern version), but what I'm
looking for as an explanation as to why one particular checkpoint would
be so bad on a low volume system, so I can appease certain management
concerns. 

This is a _really _low volume system, less than 500 writes/hour.  Normal
operation sees checkpoint related spikes of around 200-300 milliseconds.
We always checkpoint at the checkpoint timeout (every 5 minutes).
During this one checkpoint, I'm seeing transactions running 2-3 seconds.
During this time, writes are < 5/minute.

Relevant settings:
shared_buffers = 10000

checkpoint_segments = 30
checkpoint_timeout = 300

What gives?

-- 
Brad Nicholson  416-673-4106
Database Administrator, Afilias Canada Corp.



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