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?
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
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