Re: [Qemu-devel] [PATCH] Further tidy-up on block status

[Vladimir Sementsov-Ogievskiy]

14.12.2016 19:38, Alex Bligh wrote:
> Vladimir,
>
> > > +non-zero number of metadata contexts during negotiation. Servers SHOULD
> > > +reply to clients sending `NBD_CMD_BLOCK_STATUS without
> > backquote
> Fixed
>
> > > +    If zero queries are sent, then the server MUST return all
> > > +    the metadata contexts it knows about.
> > I think that 'all .. it knows about' is too much. What about 'return all 
> > available ..'? Anyway 'all ... it knows about' actually equals to 'all ... it 
> > wants'. There may be some private, or unrelated contexts, for example..
> This was not my wording, but I've changed it anyway to:
>
>      If zero queries are sent, then the server MUST return all
>      the metadata contexts that are available to        the client to select
>      on the given export with `NBD_OPT_SET_META_CONTEXT`.
>
> I think if they are available to select, we should list them. Thanks
> also for reminding me to document why I put the export name into the
> _LIST_ data (as it is for _SET_).
>
> However, this raises another question. Wouter deliberately made the
> query format freeform so that you could e.g. set a context like:
>
>     backup:modtime>201612081034
>
> which might (in theory) return a list of blocks which are newer than
> the given timestamp. It would clearly be impossible to return all such
> contexts. I wonder if we should carve out an exception here.

Interesting. Even query 'backup:modtime>*' would be a problem, not only 
empty query list.

Actually, we do not need to 'list contexts', as it is about management, 
not about data transfer. We only need a way to check, that particular 
query selects all needed contexts and no others. Just to be sure that we 
are know, what we will select by NBD_OPT_SET_META_CONTEXT with _same_ query.

So, I suggest just to say, that _LIST_ can return error if too much 
contexts are selected.|And same should be done for _SET_. And it should 
be documented that this result of query (list or error) should be equal 
for these two commands.
|


--
Best regards,
Vladimir