On Tue, Mar 24, 2009 at 7:09 PM, Mark Kirkwood <markir@paradise.net.nz> wrote:
> I'm trying to pin down some performance issues with a machine where I work,
> we are seeing (read only) query response times blow out by an order of
> magnitude or more at busy times. Initially we blamed autovacuum, but after a
> tweak of the cost_delay it is *not* the problem. Then I looked at
> checkpoints... and altho there was some correlation with them and the query
> response - I'm thinking that the raid chunksize may well be the issue.
Sounds to me like you're mostly just running out of bandwidth on your
RAID array. Whether or not you can tune it to run faster is the real
issue. This problem becomes worse as you add clients and the RAID
array starts to thrash. Thrashing is likely to be worse with a small
chunk size, so that's definitely worth a look at fixing.
> Fortunately there is an identical DR box, so I could do a little testing.
Can you try changing the chunksize on the test box you're testing on
to see if that helps?