linux-kernel - Re: [PATCH] cfq-iosched: non-rot devices do not need read queue merging

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Message-ID: <x49wrzxak5n.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 04 Jan 2010 11:51:00 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: non-rot devices do not need read queue  merging

Corrado Zoccolo <czoccolo@...il.com> writes:

> Hi Vivkek,
>
> On Mon, Jan 4, 2010 at 3:47 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
>> On Wed, Dec 30, 2009 at 11:22:47PM +0100, Corrado Zoccolo wrote:
>>> Non rotational devices' performances are not affected by
>>> distance of read requests, so there is no point in having
>>> overhead to merge such queues.
>>> This doesn't apply to writes, so this patch changes the
>>> queued[] field, to be indexed by READ/WRITE instead of
>>> SYNC/ASYNC, and only compute proximity for queues with
>>> WRITE requests.
>>>
>>
>> Hi Corrado,
>>
>> What's the reason that reads don't benefit from merging queues and hence
>> merging requests and only writes do on SSD?
>
> On SSDs, reads are just limited by the maximum transfer rate, and
> larger (i.e. merged) reads will just take proportionally longer.

This is simply not true.  You can get more bandwidth from an SSD (I just
checked numbers for 2 vendors' devices) by issuing larger read requests,
no matter whether the access pattern is sequential or random.

Cheers,
Jeff
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