Re: [Qemu-devel] [PATCH v2 0/6] Mirrored writes using blockdev-transaction

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/01/2012 03:30 PM, Eric Blake wrote:
> On 03/01/2012 02:10 PM, Anthony Liguori wrote:
>>> 2) Execute the following QMP command
>>>
>>> { "execute": "qmp_capabilities" }
>>> { "execute": "blockdev-transaction", "arguments":
>>>     {'actions': [
>>>       { 'type': 'snapshot', 'data' :
>>>         { 'device': 'ide0-hd0', 'snapshot-file':
>>> '/home/pbonzini/base.qcow2' } },
>>>       { 'type': 'mirror', 'data' :
>>>         { 'device': 'ide0-hd0', 'target': '/home/pbonzini/mirror.qcow2'
>>> } } ] } }
>>> { "execute": "cont" }
>>
>> We don't have schema introspection today.  How would one determine when
>> new transaction types are available?
>>
>> I think we need some sort of introspection method too in order for
>> clients to figure out when the command is extended.
>>
>
> I agree that introspection is necessary.  Up till now, libvirt could get
> by with query-commands (either a command exists or it doesn't).  But now
> we have the case where blockdev-transaction might exist, but doesn't
> support the particular union action such as 'mirror' that libvirt wants
> to use.
>
> Could this be something we wire up to the query-commands command?
>
> Something like:
>
> { 'type': 'CommandInfo', 'data': {'name': 'str', '*syntax': 'str'} }
> { 'command': 'query-commands,
>    'data': { '*syntax': 'bool', '*names': ['str'] },
>    'returns': ['CommandInfo'] }
>
> where the normal {"execute":"qemu-commands"} just returns the list of
> command names, but {"execute":"qemu-commands", "arguments": { "syntax":
> "true", "names" : [ "blockdev-transaction" ] } } then returns:
>
> {"return":[{"name":"blockdev-transaction", "syntax":"{ 'command' ...
> }"}], "id":"..."}
>
> that is, return back the qapi-schema.json description of the command.
> Actually, I'd guess you'd also want to be able to query 'type' listings,
> not just 'command's, so maybe this deserves a new monitor command rather
> than shoe-horning it onto an existing one.

Yes, this is what I would like to do, but it's a requires a bit of work to 
implement.  You'd have to either install qapi-schema.json and read it at run 
time or serialize it to a C data structure that can then be used to initialize a 
QObject.

Nothing Earth shattering, just a fair amount of bit moving.

Regards,

Anthony Liguori

>