Re: hanging for 30sec when checkpointing

Shane Wright <me@shanewright.co.uk> writes:
> When the db is under medium-heavy load, it periodically spawns a
> 'checkpoint subprocess' which runs for between 15 seconds and a minute.

It sounds like checkpoint is saturating your disk bandwidth.

> I've looked at the documentation and various bits about adjusting
> checkpoint segments and timings - but it seems reducing segments/timeout
> is implied to be bad, but it seems to me that increasing either will
> just make the same thing happen less often but more severely.

I was actually just about to suggest reducing the inter-checkpoint
interval.  That should reduce the volume of data that needs to be
written per checkpoint.  Might spread the pain a little thinner at least.

If you think you should have more disk bandwidth available than the
system seems to be using, maybe there is something wrong with the disk
or kernel configuration, but that's way out of my league to diagnose.
You'd probably need to find some kernel hackers to help you with that.

BTW, Jan Wieck has been doing some work recently to try to reduce the
"I/O storm at checkpoint" effect, but it won't appear till 7.5.

            regards, tom lane