On Fri, Jul 14 2017 at  1:17pm -0400,
Ewan D. Milne [off-list ref] wrote:

On Fri, 2017-07-14 at 10:19 -0400, Mike Snitzer wrote:

quoted

Do you see a benefit to extracting that portion of your WIP patch
(removing the ->complete handler entirely)?

Or leave well enough alone and just continue to disable dm-mq's ability
to stack on .request_fn paths?

Given SCSI's switch to scsi-mq by default I cannot see value in propping
up stacking on the old .request_fn devices.

So, the dm_mod.use_blk_mq flag is global, right?  I guess the question
is whether all of the block device types used on a system under DM are
supported under MQ.  If that is the case then we would be OK.

I didn't quite understand Ewan's question so we talked in person.  His
concern was whether other DM targets needed to be worried about blk-mq
vs not.  I clarified that DM multipath is the only target that is
request-based and that it is fine with stacking on scsi-mq.  And all the
bio-based targets obviously stack just fine on scsi-mq devices.

The other question is whether there are negative performance
consequences in any (corner?) cases with MQ that would result in it
being preferable to run in non-MQ mode (e.g. tag space with lpfc, did
we ever resolve that?) but the right approach there is to put the effort
into the MQ path going forward, as has been the case.

Yeap.