Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH] cfq-iosched: non-rot devices do not need read queue merging | Date | Mon, 04 Jan 2010 11:51:00 -0500 |
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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
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