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From: Jens Axboe <jens.axboe@oracle.com>
To: Paul Clements <paul.clements@steeleye.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Randy Dunlap <randy.dunlap@oracle.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler
Date: Fri, 8 Feb 2008 21:47:13 +0100	[thread overview]
Message-ID: <20080208204713.GF23197@kernel.dk> (raw)
In-Reply-To: <20080208204510.GD23197@kernel.dk>

On Fri, Feb 08 2008, Jens Axboe wrote:
> On Fri, Feb 08 2008, Paul Clements wrote:
> > Andrew Morton wrote:
> > >On Fri, 8 Feb 2008 09:33:41 -0800 Randy Dunlap <randy.dunlap@oracle.com> 
> > >wrote:
> > >
> > >>On Fri, 08 Feb 2008 11:47:42 -0500 Paul Clements wrote:
> > >>
> > >>>There have been numerous reports of problems with nbd and cfq. Deadline 
> > >>>gives better performance for nbd, anyway, so let's use it by default.
> > >
> > >Please define "problems".  If it's just "slowness" then we can live with
> > >that, but I'd hope that Jens is aware and that it's understood.
> > >
> > >It it's "hangs" or "oopses" then we panic.
> > 
> > The two problems I have experienced (which may already be fixed):
> > 
> > 1) nbd hangs with cfq on RHEL 5 (2.6.18) -- this may well have been fixed
> > 
> > There's a similar debian bug that has been filed as well:
> > 
> > http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=447638
> > 
> > 2) nbd performs about 10% better (the last time I tested) with deadline 
> > vs. cfq (the overhead of cfq doesn't provide much advantage to nbd [not 
> > being a real disk], and you end up going through the I/O scheduler on 
> > the nbd server anyway, so it makes sense that deadline is better with nbd)
> > 
> > There have been posts to nbd-general mailing list about problems with 
> > cfq and nbd also.
> 
> I'm fine with that, it's one of those things we'll do automatically when
> we have some sort of disk profile system setup. Devices without seek
> penalties should not use AS or CFQ.
> 
> Asking for a non-existing elevator is not an issue, but it may trigger
> both printks and a switch to another elevator. So if you ask for
> "deadline" and it's modular, you'll get cfq again if it's the default.
> 
> Your patch looks bad though, you forget to exit the old elevator. And
> you don't check the return value of elevator_init().
> 
> All in all, your patch definitely needs more work before it can be
> included.

argr and please don't CC closed lists on lkml, thanks! it's hugely
annoying to everyone that replies in the thread.

-- 
Jens Axboe


  reply	other threads:[~2008-02-08 20:47 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-08 16:47 Paul Clements
2008-02-08 17:33 ` Randy Dunlap
2008-02-08 18:11   ` Andrew Morton
2008-02-08 18:41     ` Paul Clements
2008-02-08 20:45       ` Jens Axboe
2008-02-08 20:47         ` Jens Axboe [this message]
2008-02-08 21:23           ` Paul Clements
2008-02-08 22:02             ` Andrew Morton
2008-02-09 13:30               ` Paul Clements
2008-02-12 23:16                 ` Andrew Morton
2008-02-18 18:16                   ` Jens Axboe
2008-02-18 23:50                     ` Andrew Morton
2008-02-19  9:19                       ` Jens Axboe
2008-02-19  9:24                         ` Jens Axboe
2008-02-19 10:02                           ` Andrew Morton
2008-02-19 10:05                             ` Jens Axboe
2008-02-08 22:45       ` [Nbd] " Mike Snitzer

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