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

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Date:	Fri, 08 Feb 2008 16:23:01 -0500
From:	Paul Clements <paul.clements@...eleye.com>
To:	Jens Axboe <jens.axboe@...cle.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

Jens Axboe wrote:
> 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.

Thanks Jens. This one should be better.

--
Paul

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