[dm-devel] multipath - unable to use multiple active paths at once, and deprecated example in docs

[Martin Wilck]

On Mon, 2019-11-25 at 13:42 -0700, Drew Hastings wrote:
> On Mon, Nov 25, 2019 at 12:48 PM Martin Wilck <mwilck at suse.de> wrote:
> > I think you are seeing this FIXME:
> > 
> > https://elixir.bootlin.com/linux/v4.19.79/source/drivers/md/dm-mpath.c#L612
> > 
> > For your testing, perhaps you just remove that if(!pgpath)
> > condition.
> > 
> > Regards,
> > Martin
> 
> That's correct, thanks. It works as expected after removing that
> condition.
> 
> With some limited testing using fio, I don't see any obvious negative
> impacts of allowing choose_pgpath to run each bio mapping. I also
> don't see any significant increase in CPU usage or IO latency during
> benchmarking.
> 
> The FIXME comment acknowledges that this sort of defeats the
> balancing function of multipath, so I'm assuming this was removed for
> a good reason. Does anyone know why this was done? In my particular
> use case, I benefit from balancing the paths for throughput, so it's
> useful to me.

This originates from the former use of dm-multipath for NVMe devices,
the now obsolete "nvme" queue mode, and from the attempt to separate
dm-multipath from SCSI and its device handler model. See 8d47e65948dd
("dm mpath: remove unnecessary NVMe branching in favor of scsi_dh
checks").

It seems indeed strange to skip choosing the path in the absence of the
SCSI device handler, which is (with a grain of salt) responsible for
switching *path groups*, not paths inside a group. It's a corner case,
I don't think many real-world multipath setups deploy bio-based dm-
mpath without a device handler.

So, I suggest you submit a patch and discuss this with Mike :-)

Regards,
Martin