Date: Fri, 18 Aug 2006 00:11:54 -0700
From: Andrew Morton <akpm@osdl.org>
To: Jens Axboe <axboe@suse.de>
Cc: David Chinner <dgc@sgi.com>, Neil Brown <neilb@suse.de>,
       linux-kernel@vger.kernel.org
Subject: Re: RFC - how to balance Dirty+Writeback in the face of slow 
 writeback.
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On Fri, 18 Aug 2006 09:03:15 +0200
Jens Axboe <axboe@suse.de> wrote:

> On Thu, Aug 17 2006, Andrew Morton wrote:
> > It seems that the many-writers-to-different-disks workloads don't happen
> > very often.  We know this because
> > 
> > a) The 2.4 performance is utterly awful, and I never saw anybody
> >    complain and
> 
> Talk to some of the people that used DVD-RAM devices (or other
> excruciatingly slow writers) on their system, and they would disagree
> violently :-)

umm, OK, I guess that has the same cause: buffer_heads from different
devices all on the same single queue.  In this case the problem is that one
device is slow.  In the same-speed-devices case the problem is that all
writeback threads get stuck on the same device, allowing others to go idle.

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