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Re: ext4 finally doing the right thing

From: Florian Weimer <fweimer(at)bfk(dot)de>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Greg Stark <stark(at)mit(dot)edu>, pgsql-performance(at)postgresql(dot)org, Jeff Davis <pgsql(at)j-davis(dot)com>
Subject: Re: ext4 finally doing the right thing
Date: 2010-01-21 14:04:25
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Lists: pgsql-performance
* Greg Smith:

> Note the comment from the first article saying "those delays can be 30
> seconds or more".  On multiple occasions, I've measured systems with
> dozens of disks in a high-performance RAID1+0 with battery-backed
> controller that could grind to a halt for 10, 20, or more seconds in
> this situation, when running pgbench on a big database.

We see that quite a bit, too (we're still on ext3, mostly 2.6.26ish
kernels).  It seems that the most egregious issues (which even trigger
the two minute kernel hangcheck timer) are related to CFQ.  We don't
see it on systems we have switched to the deadline I/O scheduler.  But
data on this is a bit sketchy.

Florian Weimer                <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH
Kriegsstraße 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99

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