Re: dm mpath: switch IO scheduler of underlying paths to "none" [was: Re: BFQ + dm-mpath]

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Sep 08 2017 at  1:07pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Fri, Sep 08 2017 at 12:48pm -0400,
> Jens Axboe <axboe@kernel.dk> wrote:
> 
> > > Please see the following untested patch.  All
> > > testing/review/comments/acks appreciated.
> > > 
> > > I elected to use elevator_change() rather than fiddle with adding a new
> > > blk-mq elevator hook (e.g. ->request_prepared) to verify that each
> > > blk-mq elevator enabled request did in fact get prepared.
> > > 
> > > Bart, please test this patch and reply with your review/feedback.
> > > 
> > > Jens, if you're OK with this solution please reply with your Ack and
> > > I'll send it to Linus along with the rest of the handful of DM changes I
> > > have for 4.14.
> > 
> > I am not - we used to have this elevator change functionality from
> > inside the kernel, and finally got rid of it when certain drivers killed
> > it. I don't want to be bringing it back.
> 
> Fine.

BTW, while I conceded "Fine": I think your justification for not
reintroducing elevator_change() lacks substance.  What is inherently
problematic about elevator_change()?

Having an elevator attached to a DM multipath device's underlying path's
request_queue just asks for trouble (especially given the blk-mq
elevator interface).

Please own this issue as a regression and help me arrive at a timely way
forward.

Thanks,
Mike