On Mon, Nov 25 2019 at 4:07pm -0500, Martin Wilck <mwilck@xxxxxxx> 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@xxxxxxx> 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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
