Re: multipath-tools: add ANA support for NVMe device

[Keith Busch <keith.busch@intel.com>]

On Mon, Nov 12, 2018 at 04:53:23PM -0500, Mike Snitzer wrote:
> On Mon, Nov 12 2018 at 11:23am -0500,
> Martin Wilck <mwilck@suse.com> wrote:
> 
> > Hello Lijie,
> > 
> > On Thu, 2018-11-08 at 14:09 +0800, lijie wrote:
> > > Add support for Asynchronous Namespace Access as specified in NVMe
> > > 1.3
> > > TP 4004. The states are updated through reading the ANA log page.
> > > 
> > > By default, the native nvme multipath takes over the nvme device.
> > > We can pass a false to the parameter 'multipath' of the nvme-core.ko
> > > module,when we want to use multipath-tools.
> > 
> > Thank you for the patch. It looks quite good to me. I've tested it with
> > a Linux target and found no problems so far.
> > 
> > I have a few questions and comments inline below.
> > 
> > I suggest you also have a look at detect_prio(); it seems to make sense
> > to use the ana prioritizer for NVMe paths automatically if ANA is
> > supported (with your patch, "detect_prio no" and "prio ana" have to be
> > configured explicitly). But that can be done in a later patch.
> 
> I (and others) think it makes sense to at least triple check with the
> NVMe developers (now cc'd) to see if we could get agreement on the nvme
> driver providing the ANA state via sysfs (when modparam
> nvme_core.multipath=N is set), like Hannes proposed here:
> http://lists.infradead.org/pipermail/linux-nvme/2018-November/020765.html
> 
> Then the userspace multipath-tools ANA support could just read sysfs
> rather than reinvent harvesting the ANA state via ioctl.

I'd prefer not duplicating the log page parsing. Maybe nvme's shouldn't
even be tied to CONFIG_NVME_MULTIPATH so that the 'multipath' param
isn't even an issue.
 
> But if we continue to hit a wall on that type of sharing of the nvme
> driver's state then I'm fine with reinventing ANA state inquiry and
> tracking like has been proposed here.
> 
> Mike
> 
> p.s. thanks for your review Martin, we really need to determine the way
> forward for full multipath-tools support of NVMe with ANA.