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

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Date:   Mon, 4 Jun 2018 08:19:21 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Mike Snitzer <snitzer@...hat.com>, Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Laurence Oberman <loberman@...hat.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        James Smart <james.smart@...adcom.com>,
        Ewan Milne <emilne@...hat.com>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Keith Busch <keith.busch@...el.com>,
        Martin George <marting@...app.com>,
        John Meneghini <John.Meneghini@...app.com>, dm-devel@...hat.com
Subject: Re: [PATCH 0/3] Provide more fine grained control over multipathing

On Wed, 30 May 2018 13:05:46 -0600
Jens Axboe <axboe@...nel.dk> wrote:

> On 5/29/18 5:27 PM, Mike Snitzer wrote:
> > On Tue, May 29 2018 at  4:09am -0400,
> > Christoph Hellwig <hch@....de> 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.
> 
> This patchset seems pretty clean and minimalist. Realistically, I'm
> guessing that SUSE and RH will ship it regardless of upstream status.
> 

Without it we're having a choice of disappointing (paying) customers or
disappointing the upstream community.

Guess.

Cheers,

Hannes