Re: [dm-devel] [PATCH 02/11] block: kill QUEUE_ORDERED_BY_TAG

[Christoph Hellwig <hch lst de>]

On Fri, Aug 13, 2010 at 04:56:32PM +0400, Vladislav Bolkhovitin wrote:
> Tejun Heo, on 08/12/2010 04:41 PM wrote:
> >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.