[libvirt] [RFC] Interface for disk hotadd/remove

Hi, I'm currently interested in implementing hard disk hot-add and -remove support for qemu (as opposed to controller-based hotplugging), and this brings up the question how to best support this feature in libvirt. Many SCSI-Controllers in real machines, for instance, allow to add and remove disks (without adding or removing the controller itself) while the system is up and running, so it would be nice to emulate this in a virtual machine. I'm focusing on qemu on the backend side, but the problem is not related to this particular backend. Rather, the question is how to integrate such a feature best into libvirt. Before implementing the functionality, it would be great to hear the community's opinion which route to take wrt. the interface. Essentially, I can see two options: - Naturally, there are virDomain{At,De}tachDevice, but the pair implements drive-hotadding via adding a new controller with an attached hard disk to the system. By extending the XML description of the drive with a parameter that specifies whether controller- or disk-based hotplugging is to be performed, it would be possible to implement the desired functionality, whilst preserving compatibility with existing semantics. Removing the drive would then require another new parameter in the XML description to identify the drive on the controller, which does not really prettify the thing. - Extend the API with a new method (for instance virDomainDiskAttach) that takes a hard disk description, a controller identifier, and a parameter that identifies the disk on the controller. - (Theoretically, it would also be possible to implement media exchange for hard disks in qemu and re-use the media exchange infrastructure already present in libvirt for CD-ROMs, but since this possibility comes to use on real hardware only very occasionally, guest operating systems are typically not really prepared to handle this well) My preference would be to go for option 2, that is, implement a new API method. Would there be any obstacles against adding such a patch to mainline? Or is anyone already working on similar functionality? Or can this be done in a much simpler way I've missed? If not, then I'd send patches for more detailed review before long. Thanks, Wolfgang