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

[Alex Bligh]

On 1 Apr 2016, at 09:35, Wouter Verhelst <address@hidden> wrote:

>> +* 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.

The MUST sentence does not cover that situation as it only refers
to completed writes.

The SHOULD sentence says that's a 'SHOULD NOT' situation in respect
of writes that have PROCESSED (i.e actioned) whether or not they
have been replied to. Of course the client has no way of knowing
whether they have been PROCESSED without a reply.

Personally I think the SHOULD clause is pretty pointless and is
unnecessary, but that's where the conversation got to n years
ago I believe.

Happy to delete the last sentence if that's wrong.

-- 
Alex Bligh