Re: [Qemu-devel] qcow2: online snasphots : internal vs external ?

[Kevin Wolf <kwolf@redhat.com>]

Am 27.08.2012 11:04, schrieb Stefan Hajnoczi:
> On Sun, Aug 26, 2012 at 10:56 AM, Alexandre DERUMIER
> <aderumier@odiso.com> wrote:
>> It is possible to achieve the same behaviour with external snapshot ? (I would like to do it online)
>> I don't see how I can rollback to the point of time of the snapshot.
> 
> The snapshot only captures the contents of the disk.  Rollback does
> not make sense without shutting down the guest.  The OS/file system
> would be very confused if the disk contents changed underneath it.
> 
> Existing hotplug can be used.  For example, if we have an external
> snapshot of a virtio-blk drive, we can use hotplug to remove the
> drive, choose the snapshot file and attach it again.  This only works
> for "data" drives, the root file system usually cannot be changed
> while the guest is running.
> 
> You may also wish to look at libvirt for higher level snapshot primitives.
> 
>> Also I see that snapshot_blkdev qmp command give in his description:
>> "Otherwise the snapshot will be internal! (currently unsupported)."
>>
>> is Live internal snapshots on the roadmap ?
> 
> I'm not aware of anyone working on adding internal snapshot in the
> near future.  Patches are welcome.

I wonder why nobody mentioned the savevm/loadvm monitor commands, which
do take an internal snapshot of a running VM. They just aren't live, and
when writing out the whole VM state this matters indeed.

However, if a disk-only snapshot is enough (this is what qemu-img
snapshot -c would produce), it would be a trivial patch to add a savevm
option to omit the VM state - and even though the snapshot is then still
not really performed in the background, it should be quick enough to be
workable.

Of course, the other thing is that external snapshots are way better
tested that internal ones. So possibly you need to put some effort in
testing and possibly fixing if you go for internal snapshots (I would
really like to see internal snapshots improved; even just that they can
be deleted without major hassle makes them the better option in many cases)

Kevin