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

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Message-ID: <a2d8d560-41d0-e17d-90df-b1953ce00a51@fb.com>
Date:   Thu, 29 Sep 2016 12:59:18 -0400
From:   Josef Bacik <jbacik@...com>
To:     Wouter Verhelst <w@...r.be>
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 09/29/2016 12:41 PM, Wouter Verhelst wrote:
> 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.
>

Huh I missed that.  Yeah that's not possible for us for sure, I think my option 
idea is the less awful way forward if we want to address that limitation.  Thanks,

Josef