From: Florian Weimer <fweimer@bfk.de>
To: Jeff <threshar@real.jefftrout.com>
Cc: Albe Laurenz <laurenz.albe@wien.gv.at>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance increase with elevator=deadline
Date: Tue, 15 Apr 2008 15:27:20 +0200
Message-ID: <82od8bi6yv.fsf@mid.bfk.de> (raw)
In-Reply-To: <C050E63A-2209-4E89-AA99-6A327CCE7380@torgo.978.org>
References: <D960CB61B694CF459DCFB4B0128514C201FA62C8@exadv11.host.magwien.gv.at>
<C050E63A-2209-4E89-AA99-6A327CCE7380@torgo.978.org>
* Jeff:
> Using 4 of these with a dataset of about 30GB across a few files
> (Machine has 8GB mem) I went from around 100 io/sec to 330 changing to
> noop. Quite an improvement. If you have a decent controller CFQ is
> not what you want. I tried deadline as well and it was a touch
> slower. The controller is a 3ware 9550sx with 4 disks in a raid10.
>
> I'll be trying this out on the big array later today. I found it
> suprising this info wasn't more widespread (the use of CFQ on a good
> controller).
3ware might be a bit special because the controller has got very deep
queues on its own, so many assumptions of the kernel I/O schedulers do
not seem to apply. Toying with the kernel/controller queue depths
might help, but I haven't done real benchmarks to see if it's actually
a difference.
A few days ago, I experienced this: On a machine with a 3ware
controller, a simple getxattr call on a file in an uncontended
directory took several minutes because a PostgreSQL dump process was
running in the background (and some other activity of a legacy
database which caused frequent fdatasync calls). Clearly, this is
unacceptable, and I've since switched to the deadline scheduler, too.
So far, this particular behavior hasn't occurred again.
--
Florian Weimer <fweimer@bfk.de>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99
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To: pgsql-performance@postgresql.org
Cc: fweimer@bfk.de, threshar@real.jefftrout.com, laurenz.albe@wien.gv.at
Subject: Re: Performance increase with elevator=deadline
In-Reply-To: <82od8bi6yv.fsf@mid.bfk.de>
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