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Date: Fri, 17 Apr 2009 16:43:51 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Cc: Andrea Righi <righi.andrea@gmail.com>, Paul Menage <menage@google.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>, agk@sourceware.org,
       akpm@linux-foundation.org, axboe@kernel.dk, baramsori72@gmail.com,
       Carl Henrik Lunde <chlunde@ping.uio.no>, dave@linux.vnet.ibm.com,
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       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/9] io-throttle documentation
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On Fri, 17 Apr 2009 15:34:53 +0800
Gui Jianfeng <guijianfeng@cn.fujitsu.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Tue, 14 Apr 2009 22:21:12 +0200
> > Andrea Righi <righi.andrea@gmail.com> wrote:
> > 
> >> +Example:
> >> +* Create an association between an io-throttle group and a bio-cgroup group
> >> +  with "bio" and "blockio" subsystems mounted in different mount points:
> >> +  # mount -t cgroup -o bio bio-cgroup /mnt/bio-cgroup/
> >> +  # cd /mnt/bio-cgroup/
> >> +  # mkdir bio-grp
> >> +  # cat bio-grp/bio.id
> >> +  1
> >> +  # mount -t cgroup -o blockio blockio /mnt/io-throttle
> >> +  # cd /mnt/io-throttle
> >> +  # mkdir foo
> >> +  # echo 1 > foo/blockio.bio_id
> > 
> > Why do we need multiple cgroups at once to track I/O ?
> > Seems complicated to me.
> 
>   Hi Kamezawa-san,
> 
>   The original thought to implement this function is for sharing a bio-cgroup
>   with other subsystems, such as dm-ioband. If the bio-cgroup is already mounted,
>   and used by dm-ioband or others, we just need to create a association between 
>   io-throttle and bio-cgroup by echo a bio-cgroup id, just like what dm-ioband does.
> 

- Why we need multiple I/O controller ? 
- Why bio-cgroup cannot be a _pure_ infrastructe as page_cgroup ?
- Why we need extra mount ?

I have no answer but, IMHO, 
 - only one I/O controller should be enabled at once.
 - bio cgroup should be tightly coupled with I/O controller and should work as
   infrastructure i.e. naming/tagging I/O should be automatically done by
   I/O controller. not by the user's hand.

Thanks,
-Kame