[dm-devel] [PATCH] dm-mpath: Improve handling of busy paths

[Bart Van Assche]

On Thu, 2017-09-21 at 06:36 +0800, Ming Lei wrote:
> Actually with GFP_ATOMIC, dispatch in dm-rq can't move on and no request
> will be dequeued from IO scheduler queue if this allocation fails, that
> means IO merge is still working at that time if the patchset of
> 'blk-mq-sched: improve SCSI-MQ performance' is applied.
> 
> As my test done, the sequential I/O performance is still not good
> even though the patchset is applied.
> 
> That isn't strange because queue depth of IO scheduler queue can be
> much bigger than either q->queue_depth or .cmd_per_lun, that means
> the underlying queue has been busy for a while before the allocation
> fails.

Hello Ming,

This patch is intended as an alternative for your patch series "dm-mpath:
improve I/O schedule". I wanted to show that it is possible to reduce
dm-mpath request submission latency if the underlying driver returns
"busy" frequently without touching the "return DM_MAPIO_DELAY_REQUEUE"
statement in multipath_clone_and_map(). I will provide measurement data as
soon as I have the time to run more measurements.

Bart.