On Fri, Aug 13, 2010 at 04:56:32PM +0400, Vladislav Bolkhovitin wrote:

Tejun Heo, on 08/12/2010 04:41 PM wrote:

quoted

Nobody is making meaningful use of ORDERED_BY_TAG now and queue
draining for barrier requests will be removed soon which will render
the advantage of tag ordering moot.

Have you seen Hannes Reinecke's and my measurements in 
http://marc.info/?l=linux-scsi&m=128110662528485&w=2 and 
http://marc.info/?l=linux-scsi&m=128111995217405&w=2 correspondingly?

If yes, what else evidences do you need to see that the tag ordering is 
a big performance win?

It's not tag odering that is a win but big queue depth.  That's what you
measured and what I fully agree on.  I haven't been able to get out of
Hannes what he actually measured.

And if you'd actually look at the patchset allowing deep queues is
exactly what it allows us, and while I haven't done testing on this
patchset but only on my previous version it does get us back to use
the full potential of large arrays exactly because of that.