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

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Message-ID: <20120410142148.GG21801@redhat.com>
Date:	Tue, 10 Apr 2012 10:21:48 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
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 Tue, Apr 10, 2012 at 03:56:39PM +0200, Jens Axboe wrote:
> 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.

Hi Jens,

Thanks. Taking a decision based on rotational flag makes sense. I am
not sure that does one get the information that a block device is a single
device or not. Especially with HBAs, SCSI Luns over Fiber, iSCSI Luns etc.
I have few Scsi Luns exported to me backed by a storage array. Everything
runs CFQ by default. And though disks in the array are rotational, they
are RAIDed and AFAIK, this information is not available to driver.

I am not sure if there is an easy way to get similar info for dm/md devices.

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

Can it be thought of as that block layer default is "deadline" and driver
can override it. Yes, I agree that it is not very clear though.

Thanks
Vivek
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