Re: [PATCH 0/3] Provide more fine grained control over multipathing

[Christoph Hellwig]

On Wed, May 30, 2018 at 06:02:06PM -0400, Mike Snitzer wrote:
> Because once nvme_core.multipath=N is set: native NVMe multipath is then
> not accessible from the same host.  The goal of this patchset is to give
> users choice.  But not limit them to _only_ using dm-multipath if they
> just have some legacy needs.

Choise by itself really isn't an argument.  We need a really good
use case for all the complexity, and so far none has been presented.

> Tough to be convincing with hypotheticals but I could imagine a very
> obvious usecase for native NVMe multipathing be PCI-based embedded NVMe
> "fabrics" (especially if/when the numa-based path selector lands).  But
> the same host with PCI NVMe could be connected to a FC network that has
> historically always been managed via dm-multipath.. but say that
> FC-based infrastructure gets updated to use NVMe (to leverage a wider
> NVMe investment, whatever?) -- but maybe admins would still prefer to
> use dm-multipath for the NVMe over FC.

That is a lot of maybes.  If they prefer the good old way on FC then
can easily stay with SCSI, or for that matter use the global switch
off.

> > This might sound stupid to you, but can't users that desperately must
> > keep using dm-multipath (for its mature toolset or what-not) just
> > stack it on multipath nvme device? (I might be completely off on
> > this so feel free to correct my ignorance).
> 
> We could certainly pursue adding multipath-tools support for native NVMe
> multipathing.  Not opposed to it (even if just reporting topology and
> state).  But given the extensive lengths NVMe multipath goes to hide
> devices we'd need some way to piercing through the opaque nvme device
> that native NVMe multipath exposes.  But that really is a tangent
> relative to this patchset.  Since that kind of visibility would also
> benefit the nvme cli... otherwise how are users to even be able to trust
> but verify native NVMe multipathing did what it expected it to?

Just look at the nvme-cli output or sysfs.  It's all been there since
the code was merged to mainline.