[dm-devel] [for-4.14 RFC PATCH 0/2] dm rq: eliminate historic blk-mq and .request_fn queue stacking restrictions

[Mike Snitzer]

On Fri, Jul 14 2017 at  3:12am -0400,
Christoph Hellwig <hch at lst.de> wrote:

> Btw, we might want to expedite this to 4.13, a 4.13 now defaults
> to blk-mq for scsi, and this patch would make sure that dm keeps
> on just working with that switch.

Don't think we need to rush anything in response to that change in
SCSI's default.  old .request_fn DM multipath works happily ontop of
blk-mq devices (so long as all paths are blk-mq).

Similarly, blk-mq DM multipath works ontop of all blk-mq devices.

Lastly, old .request_fn DM multipath ontop of old .request_fn paths
works fine.

It is just blk-mq DM multipath ontop of old .request_fn paths that is
disallowed in current upstream code.

But again, I really don't see why we should even want/need to support
that mode... hence my question raised in this RFC.