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[libvirt] libvirt + kvm external snapshots: use case?



Forgive my ignorance, but I'm trying to fill in some details for a Fedora
feature page:

https://fedoraproject.org/w/index.php?title=Features/Virt_Live_Snapshots

Yet things aren't working quite as expected in my tests. I'm trying to think
of this from an end user use case perspective. For example

- I want to test some changes to my vm, and either throw them away, or commit
them to my base disk image.
- I want to have a base disk image, but have multiple snapshots with various
mutually exclusive VM changes in each, like one snapshot with libvirt 0.7.0
installed, another with 0.8.0 installed. I want to switch between them

(maybe those assumptions are off base to begin with)

So for a running guest, I do:

virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only
virsh snapshot-create-as myvm snapshot2 "snapshot2 description" --disk-only

I make some changes that I want to discard, so I try and switch back to the
older vm and want to switch back to an older snapshot state, odo:

  virsh snapshot-revert myvm snapshot1

But I get: 'revert to external disk snapshot not supported yet'. Hmm, that
kind of invalidates all my use cases then. Additionally I don't see any API
option for merging snapshots, to fold my changes in snapshot2 into parent
snapshot1 or similar. I tried using snapshot-delete, but that also fails with
external disk snapshots: 'deletion of 1 external disk snapshots not supported yet'

Additionally, there's the --atomic option to snapshot-create-as, but it fails
for guests with more than 1 snapshotable disks: 'atomic live snapshot of
multiple disks is unsupported'. But isn't that what some of the recent qemu
additions explicitly supposed to support (that's the impression I got from jtc)?

So am I missing anything? If not, are things just not fully wired up or are
there missing chunks of functionality? And what's the use case for the current
impl of disk snapshoting (while I've read around and found lot's of info about
the implementation side of things, not too much about intended uses)?

Thanks,
Cole


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