Date:	Wed, 22 Apr 2009 12:14:43 +0900 (JST)
From:	Ryo Tsuruta <ryov@...inux.co.jp>
To:	dm-devel@...hat.com, vgoyal@...hat.com
Cc:	snitzer@...hat.com, fernando@....ntt.co.jp,
	linux-kernel@...r.kernel.org, jmoyer@...hat.com,
	jens.axboe@...cle.com, nauman@...gle.com, agk@...hat.com,
	balbir@...ux.vnet.ibm.com
Subject: Re: [dm-devel] Re: dm-ioband: Test results.

Hi,

From: Vivek Goyal <vgoyal@...hat.com>
Subject: [dm-devel] Re: dm-ioband: Test results.
Date: Tue, 21 Apr 2009 10:16:07 -0400

> On Tue, Apr 21, 2009 at 09:57:23AM -0400, Mike Snitzer wrote:
> > On Tue, Apr 21 2009 at  8:10am -0400,
> > Ryo Tsuruta <ryov@...inux.co.jp> wrote:
> > 
> > > Hi Nauman,
> > > 
> > > > > The real question is, once you create a version of dm-ioband that
> > > > > co-operates with CFQ scheduler, how that solution would compare with
> > > > > the patch set Vivek has posted? In my opinion, we need to converge to
> > > > > one solution as soon as possible, so that we can work on it together
> > > > > to refine and test it.
> > > > 
> > > > I think I can do some help for your work. but I want to continue the
> > > > development of dm-ioband, because dm-ioband actually works well and
> > > > I think it has some advantages against other IO controllers.
> > > >   - It can use without cgroup.
> > > >   - It can control bandwidth on a per partition basis.
> > > >   - The driver module can be replaced without stopping the system.
> > > 
> > > In addition, dm-ioband can run on the RHEL5.
> > 
> > RHEL5 compatibility does not matter relative to merging an I/O bandwidth
> > controller upstream.  So both the "can [be] use without cgroup" and "can
> > run on RHEL5" features do not help your cause of getting dm-ioband
> > merged upstream.  In fact these features serve as distractions.
>
> Exactly. I don't think that "it can be used without cgroup" is a feature
> or advantage. To me it is a disadvantage and should be fixed. cgroup is
> standard mechanism to group tasks arbitrarily and we should use that to make
> things working instead of coming up with own ways of grouping things and
> terming it as advantage.
> 
> What do you mean by "The driver module can be replaced without stopping
> the system"? I guess you mean that one does not have to reboot the system
> to remove ioband device? So if one decides to not use the cgroup, then
> one shall have to remove the ioband devices, remount the filesystems and
> restart the application?

Device-mapper has a feature that can replace an intermediate module
without unmount the device like the following.

      ---------------------        ---------------------
     |         /mnt        |      |        /mnt         |
     |---------------------|      |---------------------|
     | /dev/mapper/ioband1 |      | /dev/mapper/ioband1 |
     |---------------------|      |---------------------|
     |      dm-ioband      | <==> |      dm-linear      |
     |---------------------|      |---------------------|
     |      /dev/sda1      |      |      /dev/sda1      |
      ---------------------        ---------------------

So we can safely unload the dm-ioband module and update it.

> With cgroup approach, if one does not want things to be classified, a user
> can simply move all the tasks to root group and things will be fine. No
> remounting, no application stopping etc. So this also does not look like
> an advantage instead sounds like an disadvantage.
> 
> Thanks
> Vivek
> 
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