> On 15 Sep 2016, at 12:46, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Thu, Sep 15, 2016 at 12:43:35PM +0100, Alex Bligh wrote: >> Sure, it's at: >> >> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes >> >> and that link takes you to the specific section. >> >> 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. > > 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. -- Alex Bligh -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
