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

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Message-Id: <2DFDFFC6-F682-4127-8684-76CB7AF43494@alex.org.uk>
Date:   Thu, 15 Sep 2016 16:25:12 +0100
From:   Alex Bligh <alex@...x.org.uk>
To:     Eric Blake <eblake@...hat.com>
Cc:     Alex Bligh <alex@...x.org.uk>, Wouter Verhelst <w@...r.be>,
        Josef Bacik <jbacik@...com>, linux-block@...r.kernel.org,
        Markus Pargmann <mpa@...gutronix.de>, kernel-team@...com,
        "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

Eric,

> I doubt that qemu-nbd would ever want to support the situation with more
> than one client connection writing to the same image at the same time;
> the implications of sorting out data consistency between multiple
> writers is rather complex and not worth coding into qemu.  So I think
> qemu would probably prefer to just prohibit the multiple writer
> situation.

Yeah, I was thinking about a 'no multiple connection' flag.

>  And while multiple readers with no writer should be fine,
> I'm not even sure if multiple readers plus one writer can always be made
> to appear sane (if there is no coordination between the different
> connections, on an image where the writer changes AA to BA then flushes
> then changes to BB, it is still feasible that a reader could see AB
> (pre-flush state of the first sector, post-flush changes to the second
> sector, even though the writer never flushed that particular content to
> disk).

Agree

--
Alex Bligh





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