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Re: [libvirt] [RFC]: Snapshot API



2010/3/24 Daniel P. Berrange <berrange redhat com>:
> On Wed, Mar 24, 2010 at 09:09:08AM -0400, Chris Lalancette wrote:
>> On 03/24/2010 04:52 AM, Paolo Bonzini wrote:
>>
>> >
>> >> > How can<parent>  be settable? If I have snapshots A and B
>> >> >
>> >> >    A ->  B ->  current state
>> >> >
>> >> > and I create a new snapshot C, then B will be the parent of C.
>> >> >
>> >> >    A ->  B ->  C ->  current state
>> >> >
>> >> > If I create another snapshot D now and specify A to be its parent,
>> >> > what's supposed to happen then?
>> >>
>> >> You are right, that doesn't make that much sense.  I have to admit that
>> >> the tree structure is the part I thought about least, so I'll take that
>> >> part back. <parent> is just going to be an informational field about
>> >> which snapshot was current (if any) when this one was created.
>> >
>> > If discarding a snapshot also discards the children, it would definitely
>> > make sense to be able to specify the parent.
>>
>> The problem, though, is what Mattias points out; there is no (easy) way
>> that, given state C, I can get back to state A to make a new snapshot.
>> I actually have to be at state A to take a new snapshot with a parent of
>> A.  I think this is a place where we have to make it manual; if you really
>> want a new snapshot that is a child of A, you'll have to manually shutdown
>> your domain, boot to snapshot A, then take a snapshot of A.
>
> This is something virDomainCreateAtSnapshot() should solve.
>
> If you have a series
>
>   A -> B -> C
>
> And you do  virDomainCreateAtSnapshot(dom, "A"), then you get 'D'
>
>   A -> B -> C
>   |
>   \-> D

You mean you get D when you do a virDomainSnapshotCreateXML(dom, NULL)
_after_ you've done a virDomainCreateAtSnapshot(dom, "A"), don't you?

> B & C are still valid
>
> IIUC, this is how VMWare works & we essentially need our API to map to that
> since VirtualBox seems to follow the VMWare model too & there's no reason
> that QEMU can't too.
>

Yes, that's how I understand the VMware and VirtualBox snapshot model.
I'll do some testing to be really sure.

Matthias


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