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

[Stefan Hajnoczi]

On Sat, Aug 25, 2012 at 6:06 PM, Alexandre DERUMIER <address@hidden> wrote:
> I work since many years with snapshot on zfs or netapp,
> and on these system like ceph, I can rollback at the time of the snapshot, 
> and have a view of when the snapshot was taken.
>
> exemple :
> image1 : empty dir /
> take a snapshot (snap1)
> touch /file1
> now rollback to snap1
> ls /   ->empty dir, like when snap1 was taken

Using snapshot capabilities of the underlying storage is a good idea.
LVM snapshots have been used with KVM for a long time.  The same works
for zfs, btrfs, etc.  They are probably more efficient than using
QEMU's external snapshots, which must copy data between image files
when flattening the image chain.

> now,example with qemu:
> image1 : empty dir /

Did you shut down the guest here?

> take a snapshot: (qemu-img snapshot -c snap1 image1)
> touch /file1

Did you shut down the guest here?

> now rollback to snap1 (qemu-img snapshot -a snap1 image1)
> ls /file1
>
>
> the behaviour is completly different. Did I miss something ?

It should work if you shut down the guest before manipulating snapshots.

Stefan