On Fri, 11 Sep 2009, Alan McKay wrote:
Munin takes a snapshot every 5 minutes, and this graph shows it averaged over that timeframe.
The default postgresql.conf puts a checkpoint every 5 minutes as well. It's not going to be as exact as Munin's time though, they'll be just a little longer than that. I wonder if you're seeing the checkpoint pattern anyway. If the checkpoint period is fairly stable (which it could be in your case), is slighly longer than the monitoring one, and each checkpoint has the same basic shape (also usually true), each monitoring sample is going to trace out the usual checkpoint pattern by sampling a slightly later point from successive ones. The shape of the curve you're seeing seems way too close to the standard checkpoint one to be just a coincidence.
In any case, 5 minutes is unfortunately for you a really bad sampling period for a PG database because of the similarity to the checkpoint timing. You have to take measurements at least once a minute to see the checkpoints happening usefully at all. I think you're stuck generating an activity graph some other way with finer resolution to get to the bottom of this.
-- * Greg Smith gsmith@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
