Re: [Nbd] [PATCH] docs/proto: clarify NBD_CMD_FLUSH

[Wouter Verhelst <w@...112...>]

On Fri, May 08, 2015 at 12:16:16PM +0200, Paolo Bonzini wrote:
> There are two problems:
> 
> 1) A literal reading of the specification could imply that the server could
> not send a reply if fsync() fails, because in that case previous writes
> have not reached the disk.  Of course, this part of the specification only
> applies to successful replies.
> 
> 2) Flush does not apply to outstanding writes.  It applies to _completed_
> writes, ensuring that they also hit the disk.

Er, I always thought it was supposed to imply ordering as well. If you
send write request A, then write request B, then receive a reply message
for B, and then (before receiving reply for A) send a flush request,
the flush reply message should not be sent before A *and* B are
finished; that was my understanding.

Of course, this nbd server doesn't actually care either way, because it
"just" does fsync(), without disordering writes. So I suppose what
matters is the Linux kernel's semantics in this regard for flush
messages; does it care? If not, an asynchronous implementation of
nbd-server would be much simpler (because no synchronisation is
necessary); but if it does, we shouldn't change that part of the
protocol--and if we're not sure we should probably not change it either,
just to be on the safe side.

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