Message-ID: <4F8487D0.3010503@kernel.dk>
Date:	Tue, 10 Apr 2012 21:19:44 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Moyer Jeff Moyer <jmoyer@...hat.com>,
	linux-scsi@...r.kernel.org, kay.sievers@...y.org
Subject: Re: [RFC PATCH] block: Change default IO scheduler to deadline except
 SATA

On 2012-04-10 21:11, Vivek Goyal wrote:
> On Tue, Apr 10, 2012 at 08:56:19PM +0200, Jens Axboe wrote:
>> On 2012-04-10 20:53, Vivek Goyal wrote:
>>> On Tue, Apr 10, 2012 at 08:41:08PM +0200, Jens Axboe wrote:
>>>
>>> [..]
>>>>> So we are back to the question of can scsi devices find out if a Lun
>>>>> is backed by single disk or multiple disks.
>>>>
>>>> The cleanest would be to have the driver signal these attributes at
>>>> probe time. You could even adjust CFQ properties based on this, driving
>>>> the queue depth harder etc. Realistically, going forward, most fast
>>>> flash devices will be driven by a noop-like scheduler on multiqueue. So
>>>> CPU cost of the IO scheduler can mostly be ignored, since CFQ cost on
>>>> even big RAIDs isn't an issue due to the low IOPS rates.
>>>
>>> Agreed that on RAID CPU cost is not a problem. Just that idling and low
>>> queue depth kills the performance.
>>
>> Exactly, and both of these are trivially adjustable as long as we know
>> when to do it.
>>
>>> So apart from "rotational" if driver can give some hints about underlying
>>> devices being RAID (or multi device), it will help. Just that it looks
>>> like scsi does not have a way to determine that.
>>
>> This sort of thing should be done with a udev rule.
> 
> [CCing kay]
> 
> Kay does not like the idea of doing something along this line in udev.
> He thinks that kernel changes over a period of time making udev rules
> stale and hence it should be done in kernel. :-) I think he has had
> some not so good experiences in the past.
> 
> Though personally I think that anything which is not set in stone should
> go to udev. It atleast allows for easy change if user does not like the
> setting. (disable the rule, modify the rule etc). And then rules evolve
> as things change in kernel.
> 
> Anyway, this point can be detabted later once we figure out what's the
> set of atrributes to look at.

It's a bit tricky. But supposedly sysfs files are part of the ABI, no
matter how silly that may be. For these particular tunables, that means
that some parts of the ABI are only valid/there if others contain a
specific value. So I'm assuming that udev does not want to rely on that.
Now I don't know a lot about udev or udev rules, but if you could make
it depend on the value of <dev>/queue/scheduler, then it should
(supposedly) be stable and safe to rely on. It all depends on what kind
of logic you can stuff into the rules.

In any case, I'm sure that udev does not want to ship with those rules.
It would have to be a separate package. Which is fine, in my opinion.

>> It should not be too
>> hard to match for the most popular arrays, catching the majority of the
>> setups by default. Or you could ask the SCSI folks for some heuristics,
>> it's not unlikely that a few different attributes could make that bullet
>> proof, pretty much.
> 
> I am wondering what will happen to request based multipath targets in
> this scheme. There will have to be I guess additional rules to look for
> underlying paths and then change the io scheduler accordingly.

If each path is a device, each device should get caught and matched.

-- 
Jens Axboe

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