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

[Mike Snitzer]

On Mon, Nov 25 2019 at  4:07pm -0500,
Martin Wilck <mwilck at suse.de> wrote:

> 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 :-)

Hindsight being 20/20: it was misplaced micro-optimization that
compromised the broader features provided by the configurable path
selectors.

I've just removed __map_bio_fast(), see:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.5&id=dbaf971c9cdf10843071a60dcafc1aaab3162354

not sure when I'll send this to Linus.. likely in a couple weeks as part
of any other DM fixes (first batch of DM changes for 5.5 was already
merged yesterday).

Thanks,
Mike