[libvirt-users] backup disk/state

[Eric Blake]

On 03/13/2013 12:55 PM, Evaggelos Balaskas wrote:
> Hi there,
> i am trying to understand something that bothering me for some time now.
> 
> A running domain has two things that someone needs to take backup.
> So there is the disk and the state.
> 
> Using snapshots as checkpoint someone can take the disk snapshot
> but i cant understand how to "transfer" the saved state from "backup ed" disk.

In other words, you want to revert to a snapshot.

For internal snapshots, use 'virsh snapshot-revert $dom $snapname'; easy
as can be.

But libvirt is not quite there yet for external snapshots.  We have
ideas on what needs to happen, but there still needs to be some code
written to get to that point.

So for now, the answer is that you have to do it manually.

> 
> Reading through Kashyap's and Eric's posts (mostly) i can now
> understand a lot about
> backing up and snapshotting but i cant figure out (perhaps i need to
> read more) how to
> boot up the snapshot disk with the saved (?) state.

I'll start by describing an external snapshot where the memory was saved
in /path/to/memstate, and where you have only a single disk image, such
that the snapshot command made the disk have the following chain:

orig.img <- snap.img

Next, you need to decide how much you care about snap.img - do you plan
on switching branches back and forth (you need a second qcow wrapper
file), or are you discarding the work (you can go directly back to
editing orig.img, and throw away snap.img)?  If you are branching, then
orig.img must remain untouched, so the solution is to hand-create a new
qcow2 wrapper:

 qemu-img create -f qcow2 -o
backing_file=/path/to/orig.img,backing_fmt=$fmt /path/to/branch.img

with the correct $fmt according to what orig.img is, so that you now
have a chain:

          /- snap.img
orig.img <
          \- branch.img

If you have more than one disk, just be sure to do the revert for all of
the disks before reloading memory state.

Now that you know what image you want to revert back to (branch.img for
the branch case, orig.img for the discard case), you need to edit your
memory state file to reflect that.  Use 'virsh save-image-edit
/path/to/memstate' and change the <disk> elements in that screen to
point to the correct files.  (You may want to copy /path/to/memstate,
and only modify your copy, in case you goof).

Now, you can revert to the snapshot state with 'virsh restore
/path/to/memstate'.

By this point in time, your hand-edits to the external snapshot
information have probably rendered libvirt's snapshot metadata
inaccurate; you may find it useful to 'virsh snapshot-delete --metadata
$dom --$snapname' to clean up the mess in libvirt's tracking.

Hope that helps, and hope we can find time to get this coded into
libvirt itself before too much more time elapses.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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