[libvirt] ESX snapshot deletion

Chris Lalancette clalance at redhat.com
Tue Mar 30 17:49:12 UTC 2010

On 03/29/2010 07:25 PM, Matthias Bolte wrote:
> 2010/3/29 Chris Lalancette <clalance at redhat.com>:
>> Hey Mattias,
>>     I'm currently working on the semantics of
>> virDomainSnapshotDelete(), which has been renamed from
>> the earlier virDomainSnapshotDeactivate().  In qemu/kvm,
>> it seems like it is possible to delete a snapshot at any
>> time, even while that snapshot is running (though I'm not
>> yet sure exactly what happens in that case).  Talking
>> to Jirka, it seems like the virtualbox GUI doesn't allow you to
>> do any operations at all on snapshots while a domain is running,
>> although we don't know yet if this is a restriction in the
>> API as well.
>>     My questions for you are about how ESX handles this situation:
>> 1)  Can you manipulate a domain's snapshots while that domain is
>> running?
> Yes, ESX allows to freely delete snapshots and switch between them
> while the domain is running.

OK, interesting.  Virtualbox only allows you to manipulate snapshots
while the VM is shutoff (except for taking them, of course).  Qemu
seems to allow you to do stuff at runtime, though exactly what isn't
yet 100% clear.

>> 2)  If so, can you delete the snapshot that is currently running?
>> If so, what happens?
> Yes, ESX allows to delete any snapshot at any time, no restrictions.
> For example
>   base -> A -> B -> C -> current state
> C is the snapshot "that is currently running". If I delete C it gets
> merged into its parent B
>   base -> A -> B ------> current state
> and everything is fine.
> I tested this and some other scenarios, everything works as expected.
> Some words about the MERGE and DISCARD flags for virDomainSnapshotDelete:
> ESX has RemoveSnapshot and RevertToSnapshot functions. RemoveSnapshot
> merges a snapshot into its parent, so the MERGE flag is simple. In
> order to DISCARD a snapshot, you need to go back to it first using
> RevertToSnapshot and then discard it and its children (if any) using
> RemoveSnapshot.

OK, yeah.  That's a little annoying in terms of implementation, but should
fit into our API.  The question is whether this sort of usage is in line
with what users expect.  Does this API still seem reasonable?

Along those lines, I've gone back and surveyed the API's of ESX, VirtualBox,
and qemu to get a better idea if we are missing anything from our API.  I'll
be sending out another updated document with some API additions, along with
notes and questions about semantics that I compiled during my survey.

Chris Lalancette

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