Re: Selecting meta context when calling NBD_CMD_BLOCK_STATUS

[Wouter Verhelst]

Hi Vladimir,

On Fri, Dec 27, 2019 at 12:13:17PM +0000, Vladimir Sementsov-Ogievskiy wrote:
> If we are going to extent the command, I also want a possibility to use 64bit 
> length for
> commands with no payload: write_zero, discard, block_status.
> 
> I just don't remember what was the problems with extending the command 
> structure,
> we already discussed it but somehow it was not simple..
> 
> Aha, the problem is that it would be completely incompatible extension: we 
> can't implement
> it in a manner that server which don't support it will just reply EINVAL on 
> extended command,
> as it will lose the correct position in the command stream..
> 
> But I don't think it's the reason to never extend the command. Maybe, the 
> first step should
> be implementing cmd_length field feature, so, if it is negotiated, than all 
> commands are
> extended by cmd_length field which defines the length of the command in the 
> stream.

I agree that it would be useful to be able to add 64-bit commands to the
nbd_request structure at some point. Doing so could be done by having a
different magic number for the new type of the request, but only to
servers that set a transmission flag to state that they know the new
message format.

However, that is unrelated to the question that Nir asked, and so I'd
like to keep the two separate.

-- 
<Lo-lan-do> Home is where you have to wash the dishes.
  -- #debian-devel, Freenode, 2004-09-22