On Mon, Nov 30, 2015 at 01:57:24AM +0100, Lentes, Bernd wrote:
Hi,
i read that virsh uses internally qemu-img
(
http://serverfault.com/questions/692435/qemu-img-snapshot-on-live-vm).
Is that true ? so snapshotting a running vm with virsh or qemu-img is
the same ?
Functionally, it is the same.
However:
Snapshotting of guests using `virsh` will let libvirt track the
life-cycle of the snapshot and management/revert/clean-up of snapshots
is easier.
The `qemu-img` command:
$ qemu-img snapshot -c snap1 f23vm.qcow2
[And, to your question: "Will the above command harm the running VMs
filesystem ?", it depends on what is running inside your guest. To
ensure you have a consistent guest disk state, use the
qemu-guest-agent inside the guest, and supply the '--quiesce' flag
to `virsh-snapshot-create-as` command. See [1]
Maps to `virsh`:
$ virsh snapshot-create-as f23vm snap1 "Clean F23"
This results in:
- The original and the delta (the snaphot) will be stored in the
same qcow2 file (f23vm.qcow2). These type of snapshots are called
'internal snapshots' -- these are possible only with qcow2
format.
- The guest _will_ be paused briefly.
- Libvirt tracking the snapshot status of the guest.
If you want to have snapshots without any guest downtime, you can use
the more robust 'external snapshots', which operates differently: when a
snapshot is taken, a _new_ qcow2 file is created, which will track the
new guest writes from there on. And, the old image will become
read-only backing file. Some more context on it here[2].
These type of snapshots can be performed using the 'qemu-kvm-ev'
packages, which you can obtain from the oVirt repositories
http://resources.ovirt.org/pub/ovirt-master-snapshot/rpm/el7/x86_64/
Or from the CentOS build system:
http://cbs.centos.org/koji/packageinfo?packageID=539
Ideally these should be available in a repository, but I can't find one
at the moment.
[1]
http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
[2]
http://wiki.libvirt.org/page/I_created_an_external_snapshot%2C_but_libvir...
--
/kashyap