[dm-devel] [PATCH v3 0/8] dm: add request-based blk-mq support

[Keith Busch]

On Wed, 17 Dec 2014, Mike Snitzer wrote:
> As for blk-mq support... I don't have access to any NVMe hardware, etc.
> I only tested with virtio-blk (to a ramdisk, scsi-debug, device on the
> host) so I'm really going to need to lean on Keith and others to
> validate blk-mq performance.

There's a reason no one has multipath capable NVMe drives: they are not
generally available to anyone right now. :) Mine is a prototype so not
a good candidate for performance comparisons.

I was able to get my loaner back a couple hours ago though, so I built and
tested your tree and happy to say it is very successful. While running
filesystem fio, I simulated path alternating hot-removal/add sequences
and everything worked. So functionally it appears great, but I can't
speak on performance right now.

One thing with dual ported PCI-e SSDs is each path can be on a different
pci domain local to different NUMA nodes. I think there's performance
to gain if we select the target path closest to the CPU that the thread
is scheduled on. I don't have data to back that up yet, but could such
a path selection algorithm be considered in the future?