[LSF/MM ATTEND][LSF/MM TOPIC] Multipath redesign

[Mike Snitzer]

On Wed, Jan 13 2016 at 11:06am -0500,
Sagi Grimberg <sagig at dev.mellanox.co.il> wrote:

> 
> >This sounds like you aren't actually using blk-mq for the top-level DM
> >multipath queue.
> 
> Hmm. I turned on /sys/module/dm_mod/parameters/use_blk_mq and indeed
> saw a significant performance improvement. Anything else I was missing?

You can enable CONFIG_DM_MQ_DEFAULT so you don't need to manually set
use_blk_mq.

> >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."
> 
> I too see ~500K IOPs, but my nvme can push ~1500K IOPs...
> Its a simple nvme loopback [1] backed by null_blk.
> 
> [1]:
> http://lists.infradead.org/pipermail/linux-nvme/2015-November/003001.html
> http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/nvme-loop.2

OK, so you're only getting 1/3 of the throughput.  Time for us to hunt
down the bottleneck (before real devices hit it).