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

On 15 Sep 2016, at 12:46, Christoph Hellwig [off-list ref] wrote:
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On Thu, Sep 15, 2016 at 12:43:35PM +0100, Alex Bligh wrote:

quoted

Sure, it's at:
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https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-a=
nd-writes

quoted

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and that link takes you to the specific section.
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The treatment of FLUSH and FUA is meant to mirror exactly the
linux block layer (or rather how the linux block layer was a few
years ago). I even asked on LKML to verify a few points.

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Linux never expected ordering on the wire.  Before 2010 we had =

barriers

in the kernel that provided ordering to the caller, but we never
required it from the protocol / hardware.

Sure. And I think the doc section reflects exactly Linux's post-2010
expectations (note the link to the kernel documentation). IE
servers are not required to order reads/writes (save that all
writes that the server completes prior to reply to an NBD_CMD_FLUSH
must be persisted prior to the reply to that NBD_CMD_FLUSH
which I believe to be exactly the same behaviour as the kernel
dealing with an empty bio with REQ_FLUSH set).

Perhaps the section should be called "No ordering of messages
and writes"!

My point was that *in practice* disordering is not well tested
as *in practice* many server implementations do in fact process
each command in order, though that's changed recently.

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Alex Bligh