[dm-devel] [LSF/MM ATTEND] plan to deprecate old .request_fn request-based code path?

[Sagi Grimberg]

> Hi,
>
> I'd like to attend LSF/MM and as the subject covers I'd like to at least
> participate in a discussion about plans (realistic or not) for
> removing/deprecating the old .request_fn path in block core and block
> drivers.
>
> The request-based DM code (only used for multipath) has gotten more
> complex to support both old .request_fn and blk-mq (and stacking
> combinations: .request_fn ontop of blk-mq paths, blk-mq ontop of
> .request_fn paths).  Simplifying DM core in this area would be nice.
>
> One of the hurdles has been blk-mq doesn't yet have a scheduler.  I know
> Jens had/has something in the works.  But there is also the question of
> whether DM's top-level blk-mq request_queue should be trained to
> leverage/stack underlying blk-mq request_queue capabilities (Keith Busch
> was going to look at this aspect in the context of multipath on NVMe but
> I never heard anything from Keith on that).  As of now DM multipath's
> blk-mq request_queue only supports a single (virtual) hw queue.

I strongly agree we can do better here. Initial measurements of
request-based DM multipath over the (soon to come) nvme-loop driver
shows improvement from the bio-based code, but instrumentation reveals
we have plenty of room for improvement.