Re: [Qemu-devel] [PATCH v4 00/11] convert savevm to use qapi and introduce qmp command

[Pavel Hrdina <phrdina@redhat.com>]

On 10.4.2013 14:49, Eric Blake wrote:
> On 04/10/2013 06:40 AM, Luiz Capitulino wrote:
>> On Wed, 10 Apr 2013 06:24:11 -0600
>> Eric Blake <eblake@redhat.com> wrote:
>>
>>>>    - If you want to overwrite an existing snapshot, you could specify
>>>>      the 'id' or the 'name' argument or both of them and also you will
>>>>      have to use the 'force' argument
>>>
>>> But the argument made in this thread is that QMP should _not_ have a
>>> force argument.  It should be a flat-out error in QMP to try to create a
>>> snapshot with a conflicting name or tag; preferably with a distinct
>>> error type.  Higher-level apps (HMP savevm -f) would try to create; if
>>> -f is not specified, the error is good enough; if -f is specified and
>>> that particular error is returned, then HMP calls delete and then
>>> re-tries the create.  No 'force' argument needed at the QMP layer.
>>
>> To avoid adding a new error class, the HMP command could query for the
>> snapshot name and delete it if it exists before creating the snapshot.
>
> Atomic collision detection is nicer than having to pre-query - fewer QMP
> calls in the common case.  But you're right that none of the existing
> ErrorClass categories fit the idea of "creation refused because name
> would collide".
>

We still could have the force parameter for HMP because the QMP command 
will return an error message that the snapshot already exist. The only 
drawback will be that the error message will not contain an information 
that you could use a '-f' parameter for override.