14.12.2016 20:09, Wouter Verhelst wrote:
On Wed, Dec 14, 2016 at 03:08:40PM +0000, Alex Bligh wrote:
(NB: I've already applied this and pushed it)
Thanks.
* Change NBD_OPT_LIST_METADATA etc. to explicitly send a list of queries
and add a count of queries so we can extend the command later (rather than
rely on the length of option)
Sure, that works.
* For NBD_OPT_LIST_METADATA make absence of any query (as opposed to zero
length query) list all contexts, as absence of any query is now simple.
* Move definition of namespaces in the document to somewhere more appopriate.
* Various other minor changes as discussed on the mailing list
Right. I think we're getting close to a good spec now, for this thing.
One thing I've been thinking about that we might want to add:
There may be cases where a server, in performing the required calls to
be able to handle a BLOCK_STATUS request, will end up with more
information than the client asked; e.g., if the client asked information
in the base:allocation context on an extent at offset X of length Y,
then the server might conceivably do an lseek(SEEK_DATA) and/or
lseek(SEEK_HOLE). The result of that call might be that the file offset
will now point to a location Z, where Z > (X+Y).
Currently, our spec says "the sum of the *length* fields MUST not be
greater than the overall *length* of request". This means that in
essense, the server then has to throw away the information it has on the
range Z - (X + Y). In case a client was interested in that information,
that seems like a waste. I would therefore like to remove that
requirement, by rephrasing that "sum of the *length* fields" thing into
something along the following lines:
In case the server returns N extents, the sum of the *length* fields
of the first N-1 extents MUST NOT be greater than the overall *length*
of the request. The final extent MAY exceed the length of the request
if the server has that information anyway as a side effect of looking
up the required information and wishes to share it.
This would then result in the fact that the "length" field in the
BLOCK_STATUS command would be little more than a hint, since we're
saying that a server can return more data than requested (if it's
available anyway) and less data than requested (if it would be too
resource-intensive to provide all the information). Not sure whether all
that makes much sense anymore, but hey.
In addition, the combination of a server providing more information than
requested with a "REQ_ONE" flag and a length field of zero could be an
interesting way to enumerate a whole export, too. Essentially, we could
define that as a client saying "I'm interested in what the size of the
extent at offset X is, and what its properties are".
Thoughts?
Good, I'm for it. May be, in such wording there are too much information
about implementation (again, server can do it if it _wants_, is it a
side effect or not is implementation defined). In other words, "if the
server..." is better read as an example, not requirement. But it's not
important.
--
Best regards,
Vladimir
