On Wed, May 30 2018 at 3:05pm -0400, Jens Axboe <axboe@xxxxxxxxx> wrote: > On 5/29/18 5:27 PM, Mike Snitzer wrote: > > On Tue, May 29 2018 at 4:09am -0400, > > Christoph Hellwig <hch@xxxxxx> wrote: > > > >> On Tue, May 29, 2018 at 09:22:40AM +0200, Johannes Thumshirn wrote: > >>> For a "Plan B" we can still use the global knob that's already in > >>> place (even if this reminds me so much about scsi-mq which at least we > >>> haven't turned on in fear of performance regressions). > >>> > >>> Let's drop the discussion here, I don't think it leads to something > >>> else than flamewars. > >> > >> If our plan A doesn't work we can go back to these patches. For now > >> I'd rather have everyone spend their time on making Plan A work then > >> preparing for contingencies. Nothing prevents anyone from using these > >> patches already out there if they really want to, but I'd recommend > >> people are very careful about doing so as you'll lock yourself into > >> a long-term maintainance burden. > > > > Restating (for others): this patchset really isn't about contingencies. > > It is about choice. > > > > Since we're at an impasse, in the hopes of soliciting definitive > > feedback from Jens and Linus, I'm going to attempt to reset the > > discussion for their entry. > > > > In summary, we have a classic example of a maintainer stalemate here: > > 1) Christoph, as NVMe co-maintainer, doesn't want to allow native NVMe > > multipath to actively coexist with dm-multipath's NVMe support on the > > same host. > > 2) I, as DM maintainer, would like to offer this flexibility to users -- > > by giving them opt-in choice to continue using existing dm-multipath > > with NVMe. (also, both Red Hat and SUSE would like to offer this). > > > > There is no technical reason why they cannot coexist. Hence this simple > > patchset that was originally offered by Johannes Thumshirn with > > contributions from myself. > > Here's what I think - flag days tend to suck. They may be more convenient > for developers, but they inflict pain on users. Sometimes they prevent > them from moving forward, since updates are now gated on external > dependencies. Moving forward with a new architecture is great, but > proper care has to be given to existing users of multipath, regardless > of how few they may be. As I mentioned at the end of my summary (and in my first reply to this 0th header): it is on the "other" multipath toolchain to deal with switching from "native" to "other" (by writing to 'mpath_personality'). For dm-multipath, it is multipathd that would trigger the switch from "native" to "other" iff user opts-in by configuring multipath.conf to own a particular NVMe subsystem's multipathing. So users very likely won't ever have a need to write to 'mpath_personality'. And as such, they'll just default to using "native" NVMe multipath. > This patchset seems pretty clean and minimalist. Realistically, I'm > guessing that SUSE and RH will ship it regardless of upstream status. TBD really. We are keen to enable CONFIG_NVME_MULTIPATH to allow users to use native (as the default!). But we won't be able to do that unless we have this patchset. Because we really do _need_ to give our users the option of continuing to use dm-multipath. And as the Red Hat employee who would have to port it to each kernel (as long as there is a need): _please_ don't make me do that. PLEASE! ;) Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
