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Re: [Qemu-devel] [Nbd] [PATCH] Improve documentation of FUA and FLUSH

From: Wouter Verhelst
Subject: Re: [Qemu-devel] [Nbd] [PATCH] Improve documentation of FUA and FLUSH
Date: Fri, 1 Apr 2016 10:35:22 +0200
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Apr 01, 2016 at 12:03:19AM +0100, Alex Bligh wrote:
> Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA. Specifically
> the latter may be set on any command, and its semantics on commands other
> than NBD_CMD_WRITE need explaining. Further, explain how these relate to
> reordering of commands.
> Signed-off-by: Alex Bligh <address@hidden>
> ---
>  doc/proto.md | 52 ++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 42 insertions(+), 10 deletions(-)
> diff --git a/doc/proto.md b/doc/proto.md
> index c1e05c5..bc4483d 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -197,6 +197,37 @@ handle as was sent by the client in the corresponding 
> request.  In
>  this way, the client can correlate which request is receiving a
>  response.
> +#### Ordering of messages and writes
> +
> +The server MAY process commands out of order, and MAY reply out of
> +order, save that:
> +
> +* All write commands (that includes both `NBD_CMD_WRITE` and
> +  `NBD_CMD_TRIM`) that the server completes (i.e. replies to)
> +  prior to processing to a `NBD_CMD_FLUSH` MUST be written to non-volatile
> +  storage prior to replying to that `NBD_CMD_FLUSH`. The server SHOULD ensure
> +  that all write command received prior to processing the `NBD_CMD_FLUSH`
> +  (whether they are replied to or not) are written to non-volatile
> +  storage prior to processing an `NBD_CMD_FLUSH`; note this is a
> +  stronger condition than the previous 'MUST' condition. This

This seems to make little sense. Are you saying that suddenly now
sending a reply for FLUSH with outstanding writes is wrong? If not, the
above should be clarified.

< 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

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