linux-kernel - Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

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Message-ID: <20080208204713.GF23197@kernel.dk>
Date:	Fri, 8 Feb 2008 21:47:13 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Paul Clements <paul.clements@...eleye.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

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@...cle.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

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