Would it be interesting to have a command which asks the server how
much space is really used? E.g. with the new discard which can be used
by the server as a punch hole command.
Also: iirc i saw that the nbd-client sends multiple commands to the
server and then starts to wait for their replies. Shouldn't we somehow
let the client know what the queue-depth of the server is so that the
client knows how many commands it can send in advance? On the other
hand: with the new fua/flush barrier-like commands this might not be
neccessary.