[dm-devel] [LSF/MM ATTEND][LSF/MM TOPIC] Multipath redesign

[Hannes Reinecke]

On 01/13/2016 04:42 PM, Mike Snitzer wrote:
> On Wed, Jan 13 2016 at  5:50am -0500,
> Sagi Grimberg <sagig at dev.mellanox.co.il> wrote:
>
>> Another (adjacent) topic is multipath performance with blk-mq.
>>
>> As I said, I've been looking at nvme multipathing support and
>> initial measurements show huge contention on the multipath lock
>> which really defeats the entire point of blk-mq...
>>
>> I have yet to report this as my work is still in progress. I'm not sure
>> if it's a topic on it's own but I'd love to talk about that as well...
>
> This sounds like you aren't actually using blk-mq for the top-level DM
> multipath queue.  And your findings contradicts what I heard from Keith
> Busch when I developed request-based DM's blk-mq support, from commit
> bfebd1cdb497 ("dm: add full blk-mq support to request-based DM"):
>
>       "Just providing a performance update. All my fio tests are getting
>        roughly equal performance whether accessed through the raw block
>        device or the multipath device mapper (~470k IOPS). I could only push
>        ~20% of the raw iops through dm before this conversion, so this latest
>        tree is looking really solid from a performance standpoint."
>
>>> But in the end we should be able to do strip down the current (rather
>>> complex) multipath-tools to just handle topology changes; everything
>>> else will be done internally.
>>
>> I'd love to see that happening.
>
> Honestly, this needs to be a hardened plan that is hashed out _before_
> LSF and then findings presented.  It is a complete waste of time to
> debate nuance with Hannes in a one hour session.
>
> Until I implemented the above DM core changes hch and Hannes were very
> enthusiastic to throw away the existing DM multipath and multipath-tools
> code (the old .request_fn queue lock bottleneck being the straw that
> broke the camel's back).  Seems Hannes' enthusiasm hasn't tempered but
> his hand-waving is still in full form.
>
> Details matter.  I have no doubts aspects of what we have could be
> improved but I really fail to see how moving multipathing to blk-mq is a
> constructive way forward.
>
So what is your plan?
Move the full blk-mq infrastructure into device-mapper?

 From my perspective, blk-mq and multipath I/O handling have a lot 
in common (the ->map_queue callback is in effect the same ->map_rq 
does), so I still think it should be possible to leverage that directly.
But for that to happen we would need to address some of the 
mentioned issues like individual queue failures and dynamic queue 
remapping; my hope is that they'll be implemented in the course of 
NVMe over fabrics.

Also note that my proposal is more with the infrastructure 
surrounding multipathing (ie topology detection and setup), so it's 
somewhat orthogonal to your proposal.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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