On Wed, 10 Apr 2013 15:22:49 +0200 Pavel Hrdina <phrd...@redhat.com> wrote:
> 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 <ebl...@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 can add a new error class if it's important to be atomic. I don't think this is hugely important though, first because we don't support multiple QMP connections (it should work, but we don't advertise it as supported) and also because I don't think we have ever designed commands with this in mind. > 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. You're right, but we're discussing how the HMP command could detect that an existing snapshot should be deleted.
