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

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Message-Id: <C48DD624-6EDF-4EED-B474-8BEA021F00F0@alex.org.uk>
Date:   Thu, 15 Sep 2016 13:39:11 +0100
From:   Alex Bligh <alex@...x.org.uk>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alex Bligh <alex@...x.org.uk>, Wouter Verhelst <w@...r.be>,
        "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>,
        mpa@...gutronix.de, kernel-team@...com
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements


> On 15 Sep 2016, at 13:36, Christoph Hellwig <hch@...radead.org> wrote:
> 
> On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote:
>> At an implementation level that is going to be a little difficult
>> for some NBD servers, e.g. ones that fork() a different process per
>> connection. There is in general no IPC to speak of between server
>> instances. Such servers would thus be unsafe with more than one
>> connection if FLUSH is in use.
>> 
>> I believe such servers include the reference server where there is
>> process per connection (albeit possibly with several threads).
>> 
>> Even single process servers (including mine - gonbdserver) would
>> require logic to pair up multiple connections to the same
>> device.
> 
> Why?  If you only send the completion after your I/O syscall returned
> your are fine if fsync comes from a difference process, no matter
> if you're using direct or buffered I/O underneath.

That's probably right in the case of file-based back ends that
are running on a Linux OS. But gonbdserver for instance supports
(e.g.) Ceph based backends, where each connection might be talking
to a completely separate ceph node, and there may be no cache
consistency between connections.

-- 
Alex Bligh