Re: Ability to add arbitrary data to snapshots

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On Tue, Jan 26, 2016 at 12:59:00 +0300, Alexander Burluka wrote:
> On 01/25/2016 04:16 PM, Daniel P. Berrange wrote:
> > On Mon, Jan 25, 2016 at 02:55:30PM +0300, Alexander Burluka wrote:
> >> For example, we want to store suspended state of VM.
> >> I'm aware that some other careless application dealing with libvirt
> >> may erase metadata section and info about additional snapshot data will be
> >> lost.
> > What do you mean by suspended state of the VM ? Are you referring to
> > the VM memory & device state ? If so, I think that something that
> > should be explicitly represented in the API, not a opaque blob.
> We distinguish paused and suspended to disk domain states. For suspend 
> to disk we are using virDomainSaveFlags API call and stop domain.

So you are saving the domain to a file which is out of libvirt's
control and your management app (I guess) tracks the file for each
domain.

> This API call stores VM memory and device states to some file. If user 
> then calls virDomainSnapshotCreateXML, the resulted snapshot lacks
> VM memory and device states because it looks like domain is stopped and 
> it is not aware about state file. Thus, switch to this snapshot does not 
> work as expected.

It is the responsibility of your management app to associate the state
files with snapshots and to properly restore them when switching between
snapshots. I don't think libvirt should behave as a general purpose
database for management apps.

The situation would be quite different if you used virDomainManagedSave
to save its state. In this case, it's libvirt's responsibility to be
aware of a saved state when creating/reverting snapshots. And currently
libvirt doesn't handle this correctly.

Jirka

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