Re: stats collector suddenly causing lots of IO

Chris <lists(at)deksai(dot)com> writes:
> After the file was made larger and I stopped the vacuum process, I started
> seeing the problem. All other postgress processes were quiet, but the stats
> collector was constantly causing anywhere from 20-60 of the IO on the server.
> Since all the other postgres processes weren't really doing anything, and it is
> a busy web server which is predominately MySQL, I'm fairly curious as to what
> it is doing.

Yeah, the stats collector rewrites the stats file every half second, if
there have been any changes since last time --- so the bigger the file,
the more overhead. (8.4 is smarter about this, but that doesn't help
you on 8.3.)

> I straced the stats collector process. I wasn't sure what else to trace as
> there wasn't a single other postgres process doing anything.

That strace doesn't really prove much; it's what I'd expect. Here's
what to do: start a PG session, and strace that session's backend *and*
the stats collector while you manually do VACUUM some-small-table.
The VACUUM command should try to send some messages to the stats collector
process. I'm wondering if those get dropped somehow.

regards, tom lane