[libvirt] Support for qemu snapshot=on drives in libvirt

I notice that the qemu driver doesn't support snapshot drives (-drive file=foo,snapshot=on). This is important for libguestfs. Currently libguestfs hacks this using <qemu:arg>. That works fine for static disks in the libvirt XML, but lack of direct support in libvirt is a blocker for adding hotplugging to libguestfs. In qemu, the snapshot=on feature does several things: (a) It creates a temporary qcow2 disk in $TMPDIR. (b) It sets the backing file of the temporary disk to the real disk. (c) When running, writes to the disk go to the temporary overlay, and not to the real disk. (d) I believe it also lets you commit changes from the temporary disk to the backing file using a monitor command ("commit"? "save"?). However we don't need or use this feature. (e) When qemu exits, the temporary qcow2 disk is deleted. So note that when you use snapshot=on there is an implicit dependency on the $TMPDIR environment variable. I don't know of a way to create the snapshot in another directory (at least, not using snapshot=on .. possibly one can do it by creating an explicit overlay disk in libvirt). A simple implementation therefore would be to add a <snapshot/> element to <disk>. It would just add snapshot=on and ignore concerns about $TMPDIR. Or reuse the <readonly/> flag? Note that these disks are writable. A more complex implementation would create an explicit overlay disk in libvirt and clean it up afterwards. This would let us control where the temporary disk is created. XML might look like this: <snapshot [tmp=...] /> I'm also concerned about the time is takes to run the external 'qemu-img create' command, which is non-trivial in some versions of qemu. In libguestfs, every millisecond counts. $ time qemu-img create -f qcow2 test.qcow2 10M Formatting 'test.qcow2', fmt=qcow2 size=10485760 encryption=off cluster_size=65536 real 0m0.610s user 0m0.022s sys 0m0.009s Or, can this be done using existing libvirt features? Any thoughts on this before I implement something ... Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming blog: http://rwmj.wordpress.com Fedora now supports 80 OCaml packages (the OPEN alternative to F#) http://cocan.org/getting_started_with_ocaml_on_red_hat_and_fedora