From: | Alexander Staubo <alex(at)purefiction(dot)net> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Weird disk write load caused by PostgreSQL? |
Date: | 2006-10-02 17:25:12 |
Message-ID: | C8402FF1-1E6B-4D8C-BE93-509905CBBFBE@purefiction.net |
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Lists: | pgsql-general |
On Oct 2, 2006, at 17:50 , Tom Lane wrote:
> Alexander Staubo <alex(at)purefiction(dot)net> writes:
>> I have a production PostgreSQL instance (8.1 on Linux 2.6.15) that
>> seems to be writing data to disk at rates that I think are
>> disproportional to the update load imposed on the database. I am
>> looking for ways to determine the cause of this I/O.
>
> Are you sure that iostat is to be trusted?
No. :) But iostat reads directly from /dev/diskstats, which should be
reliable. Of course, it still doesn't say anything about which
process is doing the writing; for that I would need to install the
atop kernel patches or similar.
...
> The read numbers in
> particular look suspiciously uniform ... it would be a strange
> query load that would create a read demand changing less than 1%
> from hour to hour, unless perhaps that represented the disk's
> saturation point, which is not the case if you're not seeing
> obvious performance problems.
They are not uniform at all -- they correlate perfectly with the web
traffic; it just so happens that the samples I quoted were from peak
hours. Take a look at the Munin graph. (The spikes correspond to
scheduled maintenance tasks like backups.)
Alexander.
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