On Thu, May 31 2018 at 4:51am -0400, Sagi Grimberg <s...@grimberg.me> wrote:
> > >>Moreover, I also wanted to point out that fabrics array vendors are > >>building products that rely on standard nvme multipathing (and probably > >>multipathing over dispersed namespaces as well), and keeping a knob that > >>will keep nvme users with dm-multipath will probably not help them > >>educate their customers as well... So there is another angle to this. > > > >Noticed I didn't respond directly to this aspect. As I explained in > >various replies to this thread: The users/admins would be the ones who > >would decide to use dm-multipath. It wouldn't be something that'd be > >imposed by default. If anything, the all-or-nothing > >nvme_core.multipath=N would pose a much more serious concern for these > >array vendors that do have designs to specifically leverage native NVMe > >multipath. Because if users were to get into the habit of setting that > >on the kernel commandline they'd literally _never_ be able to leverage > >native NVMe multipathing. > > > >We can also add multipath.conf docs (man page, etc) that caution admins > >to consult their array vendors about whether using dm-multipath is to be > >avoided, etc. > > > >Again, this is opt-in, so on a upstream Linux kernel level the default > >of enabling native NVMe multipath stands (provided CONFIG_NVME_MULTIPATH > >is configured). Not seeing why there is so much angst and concern about > >offering this flexibility via opt-in but I'm also glad we're having this > >discussion to have our eyes wide open. > > I think that the concern is valid and should not be dismissed. And > at times flexibility is a real source of pain, both to users and > developers. > > The choice is there, no one is forbidden to use multipath. I'm just > still not sure exactly why the subsystem granularity is an absolute > must other than a volume exposed as a nvmf namespace and scsi lun (how > would dm-multipath detect this is the same device btw?) Please see my other reply, I was talking about completely disjoint arrays in my hypothetical config where having the ability to allow simultaneous use of native NVMe multipath and dm-multipath is meaningful. Mike
