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

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Message-Id: <918692FE-9B1B-4F33-8626-731F48909F41@alex.org.uk>
Date:   Thu, 15 Sep 2016 12:56:59 +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>, Josef Bacik <jbacik@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, Markus Pargmann <mpa@...gutronix.de>,
        kernel-team@...com
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

> On 15 Sep 2016, at 12:46, Christoph Hellwig <hch@...radead.org> wrote:
> 
> On Thu, Sep 15, 2016 at 12:43:35PM +0100, Alex Bligh wrote:
>> Sure, it's at:
>> 
>> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes
>> 
>> and that link takes you to the specific section.
>> 
>> The treatment of FLUSH and FUA is meant to mirror exactly the
>> linux block layer (or rather how the linux block layer was a few
>> years ago). I even asked on LKML to verify a few points.
> 
> Linux never expected ordering on the wire.  Before 2010 we had barriers
> in the kernel that provided ordering to the caller, but we never
> required it from the protocol / hardware.

Sure. And I think the doc section reflects exactly Linux's post-2010
expectations (note the link to the kernel documentation). IE
servers are not required to order reads/writes (save that all
writes that the server completes prior to reply to an NBD_CMD_FLUSH
must be persisted prior to the reply to that NBD_CMD_FLUSH
which I believe to be exactly the same behaviour as the kernel
dealing with an empty bio with REQ_FLUSH set).

Perhaps the section should be called "No ordering of messages
and writes"!

My point was that *in practice* disordering is not well tested
as *in practice* many server implementations do in fact process
each command in order, though that's changed recently.

-- 
Alex Bligh