linux-kernel - Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

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Date:	Wed, 17 Sep 2008 17:47:50 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Kazuo Ito <ito.kazuo@....ntt.co.jp>, dm-devel@...hat.com,
	jblunck@...e.de, kjamieson@...ast.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

Kazuo Ito <ito.kazuo@....ntt.co.jp> wrote:

> Write throughput to LVM snapshot origin volume is an order
> of magnitude slower than those to LV without snapshots or
> snapshot target volumes, especially in the case of sequential
> writes with O_SYNC on.
> 
> The following patch originally written by Kevin Jamieson and
> Jan Blunck and slightly modified for the current RCs by myself
> tries to improve the performance by modifying the behaviour
> of kcopyd, so that it pushes back an I/O job to the head of
> the job queue instead of the tail as process_jobs() currently
> does when it has to wait for free pages. This way, write
> requests aren't shuffled to cause extra seeks.

Did you check for starvation problems, too?

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