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

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Message-Id: <C38C69BC-CA2F-4F73-A762-CDAEA5C94702@alex.org.uk>
Date:   Thu, 15 Sep 2016 12:46:07 +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>,
        Josef Bacik <jbacik@...com>,
        "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>,
        "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 12:40, Christoph Hellwig <hch@...radead.org> wrote:
> 
> On Thu, Sep 15, 2016 at 01:29:36PM +0200, Wouter Verhelst wrote:
>> Yes, and that is why I was asking about this. If the write barriers
>> are expected to be shared across connections, we have a problem. If,
>> however, they are not, then it doesn't matter that the commands may be
>> processed out of order.
> 
> There is no such thing as a write barrier in the Linux kernel.  We'd
> much prefer protocols not to introduce any pointless synchronization
> if we can avoid it.

I suspect the issue is terminological.

Essentially NBD does supports FLUSH/FUA like this:

https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt

IE supports the same FLUSH/FUA primitives as other block drivers (AIUI).

Link to protocol (per last email) here:

https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes

-- 
Alex Bligh