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

[Vladimir Sementsov-Ogievskiy]

On 05.04.2016 16:43, Paolo Bonzini 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.
>
> Paolo

I vote for the limit too. More over, I think, there should be two sides 
limit:

1. The client can specify the limit, so server should not return more 
extents than requested. Of course, server should chose sequential 
extents from the beginning of requested range.
2. Server side limit: if client asked too many extents or not specified 
a limit at all, server should not return all extents, but only 1024 (for 
ex.) from the beginning of the range.
2.1 And/or, why not allow the server use the power of structured reply 
and send several reply chunks? Why did you forbid this? (if I correctly 
understand "This chunk type MUST appear at most once in a structured 
reply.")

--
Best regards,
Vladimir