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

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Jan 04, 2010 at 11:51:00AM -0500, Jeff Moyer wrote:
> 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.
> 

In my simple testing of 4 fio threads doing direct sequential reads
throughput varies significantly if I vary bs from 4K to 128K.

bs=4K		65MB/s
bs=128K		228MB/s

Thanks
Vivek