linux-kernel - Re: IO scheduler based IO Controller V2

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Date:	Fri, 08 May 2009 10:24:50 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Ryo Tsuruta <ryov@...inux.co.jp>
CC:	vgoyal@...hat.com, akpm@...ux-foundation.org, nauman@...gle.com,
	dpshah@...gle.com, lizf@...fujitsu.com, mikew@...gle.com,
	fchecconi@...il.com, paolo.valente@...more.it,
	jens.axboe@...cle.com, fernando@....ntt.co.jp,
	s-uchida@...jp.nec.com, taka@...inux.co.jp,
	guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	dhaval@...ux.vnet.ibm.com, balbir@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, righi.andrea@...il.com,
	agk@...hat.com, dm-devel@...hat.com, snitzer@...hat.com,
	m-ikeda@...jp.nec.com, peterz@...radead.org
Subject: Re: IO scheduler based IO Controller V2

Ryo Tsuruta wrote:
> Hi Vivek,
> 
>> Ryo, dm-ioband breaks the notion of classes and priority of CFQ because
>> of FIFO dispatch of buffered bios. Apart from that it tries to provide
>> fairness in terms of actual IO done and that would mean a seeky workload
>> will can use disk for much longer to get equivalent IO done and slow down
>> other applications. Implementing IO controller at IO scheduler level gives
>> us tigher control. Will it not meet your requirements? If you got specific
>> concerns with IO scheduler based contol patches, please highlight these and
>> we will see how these can be addressed.
> 
> I'd like to avoid making complicated existing IO schedulers and other
> kernel codes and to give a choice to users whether or not to use it.
> I know that you chose an approach that using compile time options to
> get the same behavior as old system, but device-mapper drivers can be
> added, removed and replaced while system is running.

I do not believe that every use of cgroups will end up with
a separate logical volume for each group.

In fact, if you look at group-per-UID usage, which could be
quite common on shared web servers and shell servers, I would
expect all the groups to share the same filesystem.

I do not believe dm-ioband would be useful in that configuration,
while the IO scheduler based IO controller will just work.

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