linux-kernel - Re: [RFC PATCH] block: Change default IO scheduler to deadline except SATA

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Message-ID: <4F843C17.5050004@kernel.dk>
Date:	Tue, 10 Apr 2012 15:56:39 +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>
Subject: Re: [RFC PATCH] block: Change default IO scheduler to deadline except
 SATA

On 2012-04-10 15:37, Vivek Goyal wrote:
> Hi,
> 
> I am wondering if CFQ as default scheduler is still the right choice. CFQ
> generally works well on slow rotational media (SATA?). But often
> underperforms on faster storage (storage arrays, PCIE SSDs, virtualized
> disk in linux guests etc). People often put logic in user space to tune their
> systems and change IO scheduler to deadline to get better performance on
> faster storage.
> 
> Though there is not one good answer for all kind of storage and for all
> kind of workloads, I am wondering if we can provide a better default and
> that is change default IO scheduler to "deadline" except SATA.
> 
> One can argue that some SAS disks can be slow too and benefit from CFQ. Yes,
> but default IO scheduler choice is not perfect anyway. It just tries to
> cater to a wide variety of use cases out of the box.
> 
> So I am throwing this patch out see if it flies. Personally, I think it
> might turn out to be a more reasonable default.

I think it'd be a lot more sane to just use CFQ on rotational single
devices, and default to deadline on raid or non-rotational devices. This
still isn't perfect, since less worthy SSDs still benefit from the
read/write separation, and some multi device configs will be faster as
well. But it's better.

The below patch is not a good idea. There's no clear distinction between
on what CFQ is now the default.

-- 
Jens Axboe

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