linux-kernel - Re: [Nbd] [PATCH][V3] nbd: add multi-connection support

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Date:   Thu, 29 Sep 2016 18:41:00 +0200
From:   Wouter Verhelst <w@...r.be>
To:     Josef Bacik <jbacik@...com>
Cc:     axboe@...com, linux-block@...r.kernel.org, kernel-team@...com,
        nbd-general@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [Nbd] [PATCH][V3] nbd: add multi-connection support

On Thu, Sep 29, 2016 at 10:03:50AM -0400, Josef Bacik wrote:
> So think of it like normal disks with multiple channels.  We don't send flushes 
> down all the hwq's to make sure they are clear, we leave that decision up to the 
> application (usually a FS of course).

Well, when I asked earlier, Christoph said[1] that blk-mq assumes that
when a FLUSH is sent over one channel, and the reply comes in, that all
commands which have been received, regardless of which channel they were
received over, have reched disk.

[1] Message-ID: <20160915122304.GA15501@...radead.org>

It is impossible for nbd to make such a guarantee, due to head-of-line
blocking on TCP.

[...]
> perhaps we could add a flag later that says send all the flushes down
> all the connections for the paranoid, it should be relatively
> straightforward to do.  Thanks,

That's not an unreasonable approach, I guess.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12