Subject: Re: RFC: I/O bandwidth controller
From: Fernando Luis =?ISO-8859-1?Q?V=E1zquez?= Cao 
	<fernando@oss.ntt.co.jp>
To: Ryo Tsuruta <ryov@valinux.co.jp>
Cc: taka@valinux.co.jp, dave@linux.vnet.ibm.com,
       yoshikawa.takuya@oss.ntt.co.jp, uchida@ap.jp.nec.com, ngupta@google.com,
       linux-kernel@vger.kernel.org, dm-devel@redhat.com,
       containers@lists.linux-foundation.org,
       virtualization@lists.linux-foundation.org,
       xen-devel@lists.xensource.com, agk@sourceware.org,
       righi.andrea@gmail.com
In-Reply-To: <20080808.162024.189720323.ryov@valinux.co.jp>
References: <1217870433.20260.101.camel@nimitz>
	 <1217985189.3154.57.camel@sebastian.kern.oss.ntt.co.jp>
	 <20080808.152119.43521725.taka@valinux.co.jp>
	 <20080808.162024.189720323.ryov@valinux.co.jp>
Content-Type: text/plain
Organization: NTT Open Source Software Center
Date: Fri, 08 Aug 2008 17:10:56 +0900
Message-Id: <1218183056.17648.6.camel@sebastian.kern.oss.ntt.co.jp>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1888
Lines: 40

On Fri, 2008-08-08 at 16:20 +0900, Ryo Tsuruta wrote:
> > >   - Implement a block layer resource controller. dm-ioband is a working
> > > solution and feature rich but its dependency on the dm infrastructure is
> > > likely to find opposition (the dm layer does not handle barriers
> > > properly and the maximum size of I/O requests can be limited in some
> > > cases). In such a case, we could either try to build a standalone
> > > resource controller based on dm-ioband (which would probably hook into
> > > generic_make_request) or try to come up with something new.
> > 
> > I doubt about the maximum size of I/O requests problem. You can't avoid
> > this problem as far as you use device mapper modules with such a bad
> > manner, even if the controller is implemented as a stand-alone controller.
> > There is no limitation if you only use dm-ioband without any other device
> > mapper modules.
> 
> The following is a part of source code where the limitation comes from.
> 
> dm-table.c: dm_set_device_limits()
>         /*
>          * Check if merge fn is supported.
>          * If not we'll force DM to use PAGE_SIZE or
>          * smaller I/O, just to be safe.
>          */
> 
>         if (q->merge_bvec_fn && !ti->type->merge)
>                 rs->max_sectors =
>                         min_not_zero(rs->max_sectors,
>                                      (unsigned int) (PAGE_SIZE >> 9));
> 
> As far as I can find, In 2.6.27-rc1-mm1, Only some software RAID
> drivers and pktcdvd driver define merge_bvec_fn().

Yup, exactly. The implication of this is that we may see a drop in
performance in some RAID configurations.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/