libvirt users - Feb 2018 - Re: snapshot of a raw file - how to revert ?

Hi,

i have the following system:

pc59093:~ # cat /etc/os-release
NAME="SLES"
VERSION="11.4"
VERSION_ID="11.4"
PRETTY_NAME="SUSE Linux Enterprise Server 11 SP4"
ID="sles"
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:suse:sles:11:4"

pc59093:~ # uname -a
Linux pc59093 3.0.101-84-default #1 SMP Tue Oct 18 10:32:51 UTC 2016 (15251d6)
x86_64 x86_64 x86_64 GNU/Linux

pc59093:~ # rpm -qa|grep -iE 'libvirt|kvm'
libvirt-cim-0.5.12-0.7.16
libvirt-python-1.2.5-1.102
libvirt-client-1.2.5-15.3
kvm-1.4.2-47.1
sles-kvm_en-pdf-11.4-0.33.1
libvirt-1.2.5-15.3


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 ?
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.

Thanks.


Bernd
-- 

Bernd Lentes 
Systemadministration 
Institut für Entwicklungsgenetik 
Gebäude 35.34 - Raum 208 
HelmholtzZentrum münchen 
[ mailto:bernd.lentes@helmholtz-muenchen.de | bernd.lentes@helmholtz-muenchen.de
]
phone: +49 89 3187 1241 
fax: +49 89 3187 2294 
[ http://www.helmholtz-muenchen.de/idg | http://www.helmholtz-muenchen.de/idg ] 

no backup - no mercy
 

Helmholtz Zentrum München

----- On Feb 13, 2018, at 1:38 PM, Bernd Lentes
bernd.lentes@helmholtz-muenchen.de wrote:
> Hi,
> 
> i have the following system:
> 
> pc59093:~ # cat /etc/os-release
> NAME="SLES"
> VERSION="11.4"
> VERSION_ID="11.4"
> PRETTY_NAME="SUSE Linux Enterprise Server 11 SP4"
> ID="sles"
> ANSI_COLOR="0;32"
> CPE_NAME="cpe:/o:suse:sles:11:4"
> 
> pc59093:~ # uname -a
> Linux pc59093 3.0.101-84-default #1 SMP Tue Oct 18 10:32:51 UTC 2016
(15251d6)
> x86_64 x86_64 x86_64 GNU/Linux
> 
> pc59093:~ # rpm -qa|grep -iE 'libvirt|kvm'
> libvirt-cim-0.5.12-0.7.16
> libvirt-python-1.2.5-1.102
> libvirt-client-1.2.5-15.3
> kvm-1.4.2-47.1
> sles-kvm_en-pdf-11.4-0.33.1
> libvirt-1.2.5-15.3
> 
> 
> 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
>>?
> 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.
> 
> Thanks.
> 
Hi,

i found that:
https://dustymabe.com/2015/01/11/qemu-img-backing-files-a-poor-mans-snapshotrollback/

I tried it and it seemed to work, although my root fs was checked after the
commit, anything else seemed to work.
What do you think of this procedure ?

Bernd
 

Helmholtz Zentrum München

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-kashyap-chamarthy.pdf

-- 
/kashyap

On Thu, Feb 15, 2018 at 11:41:37AM +0100, Lentes, Bernd wrote:

[...]
> Hi,
> 
> i found that:
https://dustymabe.com/2015/01/11/qemu-img-backing-files-a-poor-mans-snapshotrollback/
> 
> I tried it and it seemed to work, although my root fs was checked
> after the commit, anything else seemed to work.  What do you think of
> this procedure ?
Instead of 'qemu-img create', I'd suggest using `virsh
snapshot-create-as` (as shown in my previous email).   This will tell
libvirt to automatically use the just created QCOW2 overlay.

But yeah, one useful bit is to use the `virt-xml` tool to point to the
desired disk image (instead of `virsh edit` that I mentioned in the
previous email):

    $ virt-xml F21server --edit target=vda \
        --disk driver_type=raw,path=./A.raw


-- 
/kashyap