On 01/13/2016 04:42 PM, Mike Snitzer wrote:
On Wed, Jan 13 2016 at 5:50am -0500,
Sagi Grimberg <sagig@xxxxxxxxxxxxxxxxxx> 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
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Dr. Hannes Reinecke Teamlead Storage & Networking
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SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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