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Re: [virt-tools-list] Saved VM state not part of snapshot?



On 11/19/19 11:15 AM, Michael Weiser wrote:
> Hello Cole,
> 
> On Thu, Nov 14, 2019 at 10:43:58AM -0500, Cole Robinson wrote:
>>> Instead of handling the symptoms, might it be easier to try and make the
>>> saved memory state become part of the snapshot then? I could think of
>>> moving the .save file from the qemu/save into the
>>> qemu/snapshot/<snapshot name> directory. Or resume the saved state into
>>> a running but paused VM and snapshot that so it becomes part of the
>>> snapshot by the native qemu mechanism.
> 
>> It's possible, but I don't think there will be much energy at the
>> libvirt level to implement or review this. internal snapshots don't get
>> a lot of love as they aren't used by any of the big fancy projects like
>> ovirt or openstack, even at the qemu level they aren't particularly well
>> regarded.
> 
> So when you say, external snapshots aren't here yet, do you mean only
> virt-manager? Do the other projects simply do no snapshots at all or are
> they already employing them to such an extent that internal snapshots
> are already regarded legacy by libvirt and qemu?
> 
> So if the actual fix would be to implement external snapshots in
> virt-manager (preferably by generously borrowing what other projects are
> doing), how much work would that be and how much is there or at least on
> the roadmap already?
> 

I mean, external snapshot support at the libvirt level does not provide
the same paradigm as internal snapshots. You can create an external
snapshot (virsh snapshot-create --disk-only), but revert/run is not
supported (sudo virsh snapshot-revert), and there's no coupling with VM
RAM state to track a snapshot of a running VM.

It's possible to implement these things at the libvirt level, but it
will take a sizeable amount of work. When they are implemented, we will
need to adjust virt-manager to use them, which is likely a much smaller
chunk of work but it's hard to say until libvirt is caught up

Other projects that use snapshots just aren't using them in the same way
that virt-manager exposes AFAIK. They use them for things like live VM
storage backups. That's effectively why libvirt external snapshot
support isn't up to par with internal snapshot behavior, no big money
driving project has pushed for it to be fixed

- Cole


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