linux-kernel - Re: [Ksummit-2013-discuss] [ATTEND] scsi-mq prototype discussion

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Date:	Tue, 16 Jul 2013 14:07:29 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	James Bottomley <jbottomley@...allels.com>
Cc:	Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...nel.dk>,
	"ksummit-2013-discuss@...ts.linuxfoundation.org" 
	<ksummit-2013-discuss@...ts.linuxfoundation.org>,
	James Smart <James.Smart@...lex.Com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"kmo@...erainc.com" <kmo@...erainc.com>,
	target-devel <target-devel@...r.kernel.org>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	Tejun Heo <tj@...nel.org>, Christoph Hellwig <hch@....de>,
	"scameron@...rdog.cce.hp.com" <scameron@...rdog.cce.hp.com>
Subject: Re: [Ksummit-2013-discuss] [ATTEND] scsi-mq prototype discussion

On Sat, 2013-07-13 at 06:53 +0000, James Bottomley wrote:
> On Fri, 2013-07-12 at 12:52 +0200, Hannes Reinecke wrote:
> > On 07/12/2013 03:33 AM, Nicholas A. Bellinger wrote:
> > > On Thu, 2013-07-11 at 18:02 -0700, Greg KH wrote:
> > >> On Thu, Jul 11, 2013 at 05:23:32PM -0700, Nicholas A. Bellinger wrote:
> > >>> Drilling down the work items ahead of a real mainline push is high on
> > >>> priority list for discussion.
> > >>>
> > >>> The parties to be included in such a discussion are:
> > >>>
> > >>>   - Jens Axboe (blk-mq author)
> > >>>   - James Bottomley (scsi maintainer)
> > >>>   - Christoph Hellwig (scsi)
> > >>>   - Martin Petersen (scsi)
> > >>>   - Tejun Heo (block + libata)
> > >>>   - Hannes Reinecke (scsi error recovery)
> > >>>   - Kent Overstreet (block, per-cpu ida)
> > >>>   - Stephen Cameron (scsi-over-pcie driver)
> > >>>   - Andrew Vasquez (qla2xxx LLD)
> > >>>   - James Smart (lpfc LLD)
> > >>
> > >> Isn't this something that should have been discussed at the storage
> > >> mini-summit a few months ago?
> > > 
> > > The scsi-mq prototype, along with blk-mq (in it's current form) did not
> > > exist a few short months ago.  ;)
> > > 
> > >>  It seems very specific to one subsystem to be a kernel summit topic,
> > >> don't you think?
> > > 
> > > It's no more subsystem specific than half of the other proposals so far,
> > > and given it's reach across multiple subsystems (block, scsi, target),
> > > and the amount of off-list interest on the topic, I think it would make
> > > a good candidate for discussion.
> > > 
> > And it'll open up new approaches which previously were dismissed,
> > like re-implementing multipathing on top of scsi-mq, giving us the
> > single scsi device like other UNIX systems.
> > 
> > Also I do think there's quite some synergy to be had, as with blk-mq
> > we could nail each queue to a processor, which would eliminate the
> > need for locking.
> > Which could be useful for other subsystems, too.
> 
> Lets start with discussing this on the list, please, and then see where
> we go from there ...
> 

Yes, the discussion is beginning to make it's way to the list.  I've
mostly been waiting for blk-mq to get a wider review before taking the
early scsi-mq prototype driver to a larger public audience.

Primarily, I'm now reaching out to the people most effected by existing
scsi_request_fn() based performance limitations.  Most of them have
abandoned existing scsi_request_fn() based logic in favor of raw block
make_request() based drivers, and are now estimating the amount of
effort to move to an scsi-mq based approach.

Regardless, as the prototype progresses over the next months, having a
face-to-face discussion with the key parties in the room would be very
helpful given the large amount of effort involved to actually make this
type of generational shift in SCSI actually happen.

--nab

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