[dm-devel] awful request merge results while simulating high IOPS multipath

[Keith Busch]

On Wed, 25 Feb 2015, Mike Snitzer wrote:
> On Wed, Feb 25 2015 at  1:17pm -0500,
> Busch, Keith <keith.busch at intel.com> wrote:
>> Our first bottleneck appears to be the device mapper's single lock
>> request queue.
>
> Obviously if we switched dm-multipath over to blk-mq we'd eliminate
> that.  I'll see how things go and will share any changes I come up
> with.

Yes, I'm also looking at blk-mq. It appears conversion helps a lot.
I'm not sure though how many tags or h/w contexts to allocate to ensure
there's enough but not too many. You can't use the underlying device's
blk-tagset count (assuming you could even get to it) since those are
potentially shared among many block devices.