Re: Investigating IO Saturation

Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info> writes:
> I'm investigating a potential IO issue. We're running 7.4 on AIX 5.1.
> During periods of high activity (reads, writes, and vacuums), we are
> seeing iostat reporting 100% disk usage. I have a feeling that the
> iostat numbers are misleading. I can make iostat usage jump from less
> than 10% to greater than 95% by running a single vacuum against a
> moderate sized table (no noticeable change in the other activity).

That's not particularly surprising, and I see no reason to think that
iostat is lying to you.

More recent versions of PG include parameters that you can use to
"throttle" vacuum's I/O demand ... but unthrottled, it's definitely
an I/O hog.

The vmstat numbers suggest that vacuum is not completely killing you,
but you could probably get some improvement in foreground query
performance by throttling it back. There are other good reasons to
consider an update, anyway.

regards, tom lane