Re: [libvirt-users] snapshot of a raw file - how to revert ?
On Tue, Feb 13, 2018 at 01:38:34PM +0100, Lentes, Bernd wrote:
Hi,
[...]
I have several guests running with raw files, which is sufficent for
me. Now i'd like to snapshot one guest because i make heavy
configuration changes on it. From what i read in the net is that
libvirt supports snapshoting of raw files when the guest is shutdown
and the file of the snapshot becomes a qcow2. Right ?
The guest doesn't have to be shutdown.
First of all the term "snapshot" is a bit overloaded. There are
different kinds of snapshots. Refer slide-7 here[*].
In your case, since you want to take a snapshot of a raw disk image,
we'll be talk about "external snapshots". So let's spell out what
exactly happens when you try to take live snapshot of a raw disk image.
If you have a disk image (to which live QEMU is writing):
A.raw (live QEMU)
When you create an "external snapshot" as below:
$ virsh snapshot-create-as --domain vm1 disk-state1 \
--diskspec vda,file=/export/images/B.qcow2 \
--disk-only --atomic
You will get the following:
A.raw <-- B.qcow2 (live QEMU)
What it means is the original A.raw is turned into a read-only backing
file. A new QCOW2 "overlay" (B.qcow2) is created, and all the new
writes from now on will happen in B.qcow2.
In the above scenario, `virsh snapshot-revert` will not work (as of
now). If you now want to rever to A.raw, you have to:
(1) Shutdown the guest:
$ virsh shutdown vm1
(2) Edit the guest XML so that it now points to A.raw:
$ virsh edit vm1
And update the 'source file' attribute:
...
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/my/A.raw'/>
...
(3) Start the guest again:
$ virsh start vm1
* * *
If you want to take backups, there's a cleaner way:
https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
The above is possible with QEMU 2.1 (and above), libvirt-1.2.9 (and
above). So, in your environment, you have to upgrade.
I try to avoid converting my raw file to a qcow2 file. I can
shutdown
the guest for a certain time, that's no problem. I don't need a live
snapshot. But how can i revert to my previous state if my
configuration changes go wrong ? Can i do this with snapshot-revert
or do i have to edit the xml file and point the hd again to the origin
raw file ? What i found in the net wasn't complete clear.
I answered your questions above to keep them in one place. (Yes, you're
right — you have to edit the XML file and point to the original disk.)
The command `virsh snapshot-revert` works only with "internal snapshots"
— where your original and the delta, all of them reside in the single
QCOW2 file.
PS: I also fixed the quoting in your email, it was a bit off.
[*]
https://kashyapc.fedorapeople.org/virt/lc-2012/lceu-2012-virt-snapshots-k...
--
/kashyap