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

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Date:   Thu, 15 Sep 2016 05:36:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Alex Bligh <alex@...x.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>, 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 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.