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Date:   Fri, 25 May 2018 16:30:58 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Johannes Thumshirn <jthumshirn@suse.de>
Cc:     Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Hannes Reinecke <hare@suse.de>,
        Mike Snitzer <snitzer@redhat.com>,
        Laurence Oberman <loberman@redhat.com>,
        Ewan Milne <emilne@redhat.com>,
        James Smart <james.smart@broadcom.com>,
        Linux Kernel Mailinglist <linux-kernel@vger.kernel.org>,
        Linux NVMe Mailinglist <linux-nvme@lists.infradead.org>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Martin George <marting@netapp.com>,
        John Meneghini <John.Meneghini@netapp.com>
Subject: Re: [PATCH 0/3] Provide more fine grained control over multipathing
Message-ID: <20180525143058.GA26391@lst.de>
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On Fri, May 25, 2018 at 04:22:17PM +0200, Johannes Thumshirn wrote:
> But Mike's and Hannes' arguments where reasonable as well, we do not
> know if there are any existing setups we might break leading to
> support calls, which we have to deal with. Personally I don't believe
> there are lot's of existing nvme multipath setups out there, but who
> am I to judge.

I don't think existing setups are very likely, but they absolutely
are a valid reason to support the legacy mode.  That is why we support
the legacy mode using the multipath module option.  Once you move
to a per-subsystem switch you don't support legacy setups, you
create a maze of new setups that we need to keep compatibility
support for forever.

> So can we find a middle ground to this? Or we'll have the
> all-or-nothing situation we have in scsi-mq now again. How about
> tieing the switch to a config option which is off per default?

The middle ground is the module option.  It provides 100% backwards
compatibility if used, but more importantly doesn't create hairy
runtime ABIs that we will have to support forever.