On Thu, 2007-11-29 at 16:14 +0000, Simon Riggs wrote:
> On Thu, 2007-11-29 at 10:10 -0500, Brad Nicholson wrote:
> > 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?
>
> If the timing is regular, its most likely a human-initiated action
> rather then a behavioural characteristic.
>
> VACUUM runs in background at that time, updates loads of blocks which
> need to be written out at checkpoint time. That slows queries down at
> that time but not others.
Bingo. Big vacuum daily vacuum completes shortly before this chckpoint.
Thanks.
--
Brad Nicholson 416-673-4106
Database Administrator, Afilias Canada Corp.
