linux-kernel - Re: Regarding dm-ioband tests

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Date:	Tue, 08 Sep 2009 12:01:19 +0900 (JST)
From:	Ryo Tsuruta <ryov@...inux.co.jp>
To:	riel@...hat.com
Cc:	vgoyal@...hat.com, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, jens.axboe@...cle.com, agk@...hat.com,
	akpm@...ux-foundation.org, nauman@...gle.com,
	guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	balbir@...ux.vnet.ibm.com
Subject: Re: Regarding dm-ioband tests

Hi Rik,

Rik van Riel <riel@...hat.com> wrote:
> Ryo Tsuruta wrote:
> 
> > However, if you want to get fairness in a case like this, a new
> > bandwidth control policy which controls accurately according to
> > assigned weights can be added to dm-ioband.
> 
> Are you saying that dm-ioband is purposely unfair,
> until a certain load level is reached?

Not unfair, dm-ioband(weight policy) is intentionally designed to
use bandwidth efficiently, weight policy tries to give spare bandwidth
of inactive groups to active groups.

> > We regarded reducing throughput loss rather than reducing duration
> > as the design of dm-ioband. Of course, it is possible to make a new
> > policy which reduces duration.
> 
> ... while also reducing overall system throughput
> by design?

I think it reduces system throughput compared to the current
implementation, because it causes more overhead to do fine grained
control. 

> Why are you even bothering to submit this to the
> linux-kernel mailing list, when there is a codebase
> available that has no throughput or fairness regressions?
> (Vivek's io scheduler based io controler)

I think there are some advantages to dm-ioband. That's why I post
dm-ioband to the mailing list.

- dm-ioband supports not only proportional weight policy but also rate
  limiting policy. Besides, new policies can be added to dm-ioband if
  a user wants to control bandwidth by his or her own policy.
- The dm-ioband driver can be replaced without stopping the system by
  using device-mapper's facility. It's easy to maintain.
- dm-ioband can use without cgroup. (I remember Vivek said it's not an
  advantage.)

Thanks,
Ryo Tsuruta
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