Re: [Qemu-devel] Adding an overlay with 'blockdev-add' fails with: "Parameter 'driver' is missing"

[Eric Blake]

On 02/23/2017 10:33 AM, Kashyap Chamarthy wrote:
> On Thu, Feb 23, 2017 at 04:11:21PM +0100, Markus Armbruster wrote:
>> Kashyap Chamarthy <address@hidden> writes:
>>
>>> TL;DR: Using `blockdev-add` to create an overlay file succeeds with QEMU
>>>        2.7.0, however the *same* test fails with QEMU from Git (plus a
>>>        couple of patch series from mailing list).  Details below.
>>
>> commit 0153d2f50bc2ad3f41810d838fcf66acbf10f07a
>> Author: Kevin Wolf <address@hidden>
>> Date:   Fri Oct 7 17:05:04 2016 +0200
>>
>>     block: Remove "options" indirection from blockdev-add
> 
> [...]
> 
>> Sorry for the confusion!  Early adopters always have the most fun ;)
> 
> Ah, I see, thanks for the pointer.
> 
> I tried to come up with the below JSON, after looking at the QAPI schema
> documentation (from qemu/qapi/block-core.json), still I seem to be
> tripping up somewhere.  Now it doesn't recognize the 'backing' option:
> 
> $ socat UNIX:./qmp-sock READLINE,history=$HOME/.qmp_history,prompt='QMP> '
> [...]
> QMP> {
> QMP>   "execute":"blockdev-add",
> QMP>   "arguments":{
> QMP>     "driver":"qcow2",
> QMP>     "node-name":"node1"
> QMP>   },

Oops, you ended your arguments too soon.

> QMP>   "file":{
> QMP>     "driver":"file",
> QMP>     "filename":"/export/target.qcow2"
> QMP>   },
> QMP>   "backing":{

Here, you passed "file" and "backing" as siblings of "execute", but they
should be children of "arguments".

> QMP>     "driver":"qcow2",
> QMP>     "file":{
> QMP>       "driver":"file",
> QMP>       "filename":"/export/base.qcow2"
> QMP>     }
> QMP>   }
> QMP> }
> {"error": {"class": "GenericError", "desc": "QMP input object member 
> 'backing' is unexpected"}}
> QMP> 

Try (untested on my end) this rewrite of your original post (just
removing the '"options":{' and corresponding '}' from that listing):

{
    "execute": "blockdev-add",
    "arguments": {
            "backing": "virtio0",
            "driver": "qcow2",
            "id": "virtio1",
            "file": {
                "driver": "file",
                "filename": "/export/target.qcow2"
            }
    }
}

or this attempt at your current post:

{   "execute":"blockdev-add",
    "arguments":{
     "driver":"qcow2",
     "node-name":"node1"
     "file":{
      "driver":"file",
      "filename":"/export/target.qcow2"
     },
     "backing":{
      "driver":"qcow2",
      "file":{
        "driver":"file",
        "filename":"/export/base.qcow2"
      }
     }
    }
}

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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