[libvirt] Proposed functionality: API to extract host description from saved state, restore with modified XML

[Charles Duffy]

Per subject. Obviously only for use by folks who *really* know what 
they're doing. :)

I'm looking for the ability to restore a snapshot saved with 
domain.save(), with disk images pointing at a different location (in my 
particular use case, pointing at a an empty copy-and-write image 
backending into a snapshot of the old disk state). Further, as I may run 
multiple copies of the same host at once (from the same initial state, 
against different copy-on-write disk images backending into the same 
read-only store, into different disconnected networks), the UUID as well 
as the name may need to be changed.

I could just rewrite the header of the saved-state file, but this has a 
few disadvantages:
  - To mimic the copy-on-write behavior of the disk images, I would need 
to copy the entire saved-domain file. I'm trying to minimize the time 
and disk penalty of snapshot/restore, so this is suboptimal.
  - My code is dependent on the file format used by the particular 
backend, and may break when that backend is revved.

Would a patch adding API calls to
  - return the XML description of a saved host, given the name
  - restore a saved host with a different description
be considered welcome?

I presume that to maintain backwards compatibility, the latter would be 
expected to be exposed to clients via a new call, rather than adding a 
parameter to virDomainRestore(); in communicating with the drivers, on 
the other hand, my first instinct would be to add an extra parameter to 
virDrvDomainRestore.

Thanks!