On Wed, Jun 04, 2014 at 10:50:53AM -0400, Tejun Heo wrote:

Hmmm... the biggest thing is ioscheds.  They heavily rely on being
strongly synchronized and are pretty important for rotating rusts.
Maybe they can be made to work with blk-mq by forcing single queue or
something but do we wnat that?

Jens is planning to add an (optional) I/O scheduler to blk-mq, and
that is indeed required for proper disk support.  I don't think there
even is a need to limit it to a single queue technically, although
devices that support multiple queues are unlikely to need I/O
scheduling.