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

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Date:   Thu, 15 Sep 2016 05:18:18 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Alex Bligh <alex@...x.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>, Wouter Verhelst <w@...r.be>,
        "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>, linux-block@...r.kernel.org,
        Josef Bacik <jbacik@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        mpa@...gutronix.de, kernel-team@...com
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

On Thu, Sep 15, 2016 at 01:11:24PM +0100, Alex Bligh wrote:
> > NBD_CMD_FLUSH (3)
> > 
> > A flush request; a write barrier. 
> 
> I can see that's potentially confusing as isn't meant to mean 'an old-style
> linux kernel block device write barrier'. I think in general terms it
> probably is some form of barrier, but I see no problem in deleting the
> words "a write barrier" from the spec text if only to make it
> clearer.

Yes, please do that.  A "barrier" implies draining of the queue.

> However, I think the description of the command itself:
> 
> > The server MUST NOT send a successful reply header for this request before all write requests for which a reply has already been sent to the client have reached permanent storage (using fsync() or similar).
> 
> and the ordering section I pointed you to before, were both correct, yes?

Yes, this seems correct.

> actually fdatasync() technically does more than is necessary, as it
> will also flush commands that have been processed, but for which no
> reply has yet been sent - that's no bad thing.

Yes.  But without an actual barrier it's hard to be exact - and
fdatasync does the right thing by including false positives.