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

[Jeff Moyer <jmoyer@redhat.com>]

Corrado Zoccolo <czoccolo@gmail.com> writes:

> Hi Vivkek,
>
> On Mon, Jan 4, 2010 at 3:47 PM, Vivek Goyal <vgoyal@redhat.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