linux-kernel - Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

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Date:   Thu, 15 Sep 2016 14:26:31 +0200
From:   Wouter Verhelst <w@...r.be>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>, linux-block@...r.kernel.org,
        Josef Bacik <jbacik@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Markus Pargmann <mpa@...gutronix.de>, kernel-team@...com
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

On Thu, Sep 15, 2016 at 05:20:08AM -0700, Christoph Hellwig wrote:
> On Thu, Sep 15, 2016 at 02:01:59PM +0200, Wouter Verhelst wrote:
> > Yes. There was some discussion on that part, and we decided that setting
> > the flag doesn't hurt, but the spec also clarifies that using it on READ
> > does nothing, semantically.
> >
> > 
> > The problem is that there are clients in the wild which do set it on
> > READ, so it's just a matter of "be liberal in what you accept".
> 
> Note that FUA on READ in SCSI and NVMe does have a meaning - it
> requires you to bypass any sort of cache on the target.  I think it's an
> wrong defintion because it mandates implementation details that aren't
> observable by the initiator, but it's still the spec wording and nbd
> diverges from it.  That's not nessecarily a bad thing, but a caveat to
> look out for.

Yes. I think the kernel nbd driver should probably filter out FUA on
READ. It has no meaning in the case of nbd, and whatever expectations
the kernel may have cannot be provided for by nbd anyway.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12