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

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Message-ID: <20161003075701.GA29457@infradead.org>
Date:   Mon, 3 Oct 2016 00:57:01 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Wouter Verhelst <w@...r.be>
Cc:     Christoph Hellwig <hch@...radead.org>, Josef Bacik <jbacik@...com>,
        Alex Bligh <alex@...x.org.uk>, Jens Axboe <axboe@...com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Nbd] [PATCH][V3] nbd: add multi-connection support

On Mon, Oct 03, 2016 at 09:51:49AM +0200, Wouter Verhelst wrote:
> Actually, I was pointing out the TCP head-of-line issue, where a delay
> on the socket that contains the flush reply would result in the arrival
> in the kernel block layer of a write reply before the said flush reply,
> resulting in a write being considered part of the flush when in fact it
> was not.

The kernel (or any other user of SCSI/ATA/NVMe-like cache flushes)
will wait for all I/O that needs to be in the cache for explicitly,
so this is not a problem.

> Can you clarify what you mean by that? Why is it an "odd flush
> definition", and how would you "properly" define it?

E.g. take the defintion from NVMe which also supports multiple queues:

"The Flush command shall commit data and metadata associated with the
specified namespace(s) to non-volatile media. The flush applies to all
commands completed prior to the submission of the Flush command.
The controller may also flush additional data and/or metadata from any
namespace."

The focus is completed - we need to get a reply to the host first
before we can send the flush command, so anything that we require
to be flushed needs to explicitly be completed first.