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

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Date:   Tue, 29 May 2018 09:18:55 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Laurence Oberman <loberman@...hat.com>,
        Ewan Milne <emilne@...hat.com>,
        James Smart <james.smart@...adcom.com>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Martin George <marting@...app.com>,
        John Meneghini <John.Meneghini@...app.com>
Subject: Re: [PATCH 0/3] Provide more fine grained control over multipathing

On Mon, 28 May 2018 23:02:36 -0400
Mike Snitzer <snitzer@...hat.com> wrote:

> On Mon, May 28 2018 at  9:19pm -0400,
> Martin K. Petersen <martin.petersen@...cle.com> wrote:
> 
> > 
> > Mike,
> > 
> > I understand and appreciate your position but I still don't think
> > the arguments for enabling DM multipath are sufficiently
> > compelling. The whole point of ANA is for things to be plug and
> > play without any admin intervention whatsoever.
> > 
> > I also think we're getting ahead of ourselves a bit. The assumption
> > seems to be that NVMe ANA devices are going to be broken--or that
> > they will require the same amount of tweaking as SCSI devices--and
> > therefore DM multipath support is inevitable. However, I'm not sure
> > that will be the case.
> >   
> > > Thing is you really don't get to dictate that to the industry.
> > > Sorry.  
> > 
> > We are in the fortunate position of being able to influence how the
> > spec is written. It's a great opportunity to fix the mistakes of
> > the past in SCSI. And to encourage the industry to ship products
> > that don't need the current level of manual configuration and
> > complex management.
> > 
> > So I am in favor of Johannes' patches *if* we get to the point
> > where a Plan B is needed. But I am not entirely convinced that's
> > the case just yet. Let's see some more ANA devices first. And once
> > we do, we are also in a position where we can put some pressure on
> > the vendors to either amend the specification or fix their
> > implementations to work with ANA.  
> 
> ANA really isn't a motivating factor for whether or not to apply this
> patch.  So no, I don't have any interest in waiting to apply it.
> 
Correct. That patch is _not_ to work around any perceived incompability
on the OS side.
The patch is primarily to give _admins_ a choice.
Some installations like hosting providers etc are running quite complex
scenarios, most of which are highly automated.
So for those there is a real benefit to be able to use dm-multipathing
for NVMe; they are totally fine with having a performance impact if
they can avoid to rewrite their infrastructure.

Cheers,

Hannes