[dm-devel] dm-mpath: Work with blk multi-queue drivers

[Mike Snitzer]

On Wed, Sep 24 2014 at  2:34pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> 
> I never did take the time to properly review Hannes' proposal but now
> that you're floating this blk-mq support for DM core (and DM mpath) I'm
> clearly going to have to take this all on in a much more focused way.
> 
> Christoph/Hannes/Junichi/Keith/others, can you see a way forward that
> offers a lighter request-based DM that makes required callouts to (new?)
> block interfaces that helps us abstract the old request and blk-mq
> request allocation, etc?

(sorry about replying to myself...)

SO revisiting that thread from above, these posts stand out:
http://www.redhat.com/archives/dm-devel/2014-June/msg00026.html
http://www.redhat.com/archives/dm-devel/2014-June/msg00028.html

I'd love to see us get rid of request-based DM's bio cloning for each
cloned request (we never did get an answer from the NEC guys to know
_why_ that was done).

http://www.redhat.com/archives/dm-devel/2014-June/msg00029.html

But I now see what Christoph was saying about needing the call
blk_get_request() against the low level path... and that completely
avoiding request cloning like Hannes did is a non-starter for blk-mq.

So if we could:
1) rip out the rq-based DM's cloning of all bios in a request
2) rebase Keith's approach ontop of 1) then we could go from there
   - but happy to put more thought in upfront to avoid busy work; and
     I'd encourage everyone else to do the same...