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Re: High load and iowait but no disk access

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Rémy Beaumont <remyb(at)medrium(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: High load and iowait but no disk access
Date: 2005-08-30 16:29:02
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Lists: pgsql-performance
=?ISO-8859-1?Q?R=E9my_Beaumont?= <remyb(at)medrium(dot)com> writes:
> On 30-Aug-05, at 12:15, Tom Lane wrote:
>> I know zip about NetApps, but doesn't the 8th column indicate pretty
>> steady disk reads?

> Yes, but they are very low.

Sure, but that's more or less what you'd expect if the thing is randomly
seeking all over the disk :-(.  Just because it's a NetApp doesn't mean
it's got zero seek time.

You did not say what sort of query this is, but I gather that it's doing
an indexscan on a table that is not at all in index order.  Possible
solutions involve reverting to a seqscan (have you forced the planner to
choose an indexscan here, either directly or by lowering random_page_cost?)
or CLUSTERing the table by the index (which would need to be repeated
periodically, so it's not a great answer).

			regards, tom lane

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