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

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Date:	Fri, 08 Feb 2008 13:41:20 -0500
From:	Paul Clements <paul.clements@...eleye.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org, nbd-general@...ts.sourceforge.net,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

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.

--
Paul

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