Re: [Qemu-devel] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extension

[Paolo Bonzini]

On 07/04/2016 17:35, Pavel Borzenkov wrote:
> > On 05/04/2016 06:05, Kevin Wolf wrote:
> > > The options I can think of is adding a request field "max number of
> > > descriptors" or a flag "only single descriptor" (with the assumption
> > > that clients always want one or unlimited), but maybe you have a better
> > > idea.
> > 
> > I think a limit is better.  Even if the client is ultimately going to
> > process the whole file, it may take a very long time and space to
> > retrieve all the descriptors in one go.  Rather than query e.g. 16GB at
> > a time, I think it's simpler to put a limit of 1024 descriptors or so.
> 
> Agree. I'm not sure that the value of the limit should be hard-coded
> into the protocol, though. Why don't just allow a server to send less
> data than requested (similar to what SCSI "GET LBA STATUS" allows) and
> allow the server to choose the limit suitable for it (without directly
> exposing it to the clients)?

Yes, definitely.  The number was just an example.  Even 1 would be fine.

Paolo