Re: [Qemu-devel] [PATCH 2/3] block: Add QMP support for streaming to an intermediate layer

[Alberto Garcia]

On Thu, Mar 05, 2015 at 03:09:58PM +0100, Kevin Wolf wrote:

> >  { 'command': 'block-stream',
> > -  'data': { 'device': 'str', '*base': 'str', '*backing-file': 'str',
> > -            '*speed': 'int', '*on-error': 'BlockdevOnError' } }
> > +  'data': { 'device': 'str', '*base': 'str', '*top': 'str',
> > +            '*backing-file': 'str', '*speed': 'int',
> > +            '*on-error': 'BlockdevOnError' } }
> 
> There is no point in specifying some root node as 'device' that
> isn't actually involved in the operation; worse, it isn't even
> possible in the general case because 'top' could have multiple
> users/parents.
> 
> A better interface would probably be to allow node names for
> 'device' and leave everything else as it is.

Ok, I changed the code and it does make the implementation simpler.

One issue that I'm finding is that when we move the block-stream
job to an intermediate node, where the device name is empty, we get
messages like "Device '' is busy".

I can use node names instead, but they are also not guaranteed to
exist. I heard there was a plan to auto-generate names, and searching
the archives I found this patch by Jeff Cody:

http://lists.gnu.org/archive/html/qemu-devel/2014-06/msg04057.html

But it seems that it was never merged?

If we are going to have a scenario where a parameter can mean either a
device or a node name, we need a clear way to identify that node.

Berto