Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request)

[Prof. Dr. Michael Schefczyk]

Dear Eric, dear all,

Again, thank you very much. I now gather that I took the wrong path towards 
nightly backups of running VM. I remain surprised that I did work for a 
relatively long time.

A major book on KVM in German language by Kofler & Spenneberg recommends the 
following approach for online backups (with and without "--disk-only"):

virsh snapshot-create-as vm XXX --disk-only
qemu-img convert -f qcow2 -s XXX -O qcow2 XXX.img /backup/YYY.img
virsh snapshot-delete vm XXX

Would this be any better than my script, because it uses virsh 
snapshot-create-as instead of qemu-img snapshot? The second command is still 
qemu-img convert which may be problematical.

The problem I am facing is that the documentation is not easy to understand for 
the average user/administrator who is not among the kvm developers and experts. 
I have of course tried to read section 14.2.3 of RHEL 7 Virtualization 
Deployment and Administration Guide on backups, but I found that too cryptic 
for someone like myself to draw practical consequences from it.

Regards,

Michael
___________________________________________________________ 
Technische Universität Dresden
Fakultät Wirtschaftswissenschaften
Lehrstuhl für Entrepreneurship und Innovation
Prof. Dr. Michael Schefczyk
D-01062 Dresden 
 
Fon: +49-3 51-4 63-3 68 81 
Fax: +49-3 51-4 63-3 68 83 
www.gruenderlehrstuhl.de






-----Ursprüngliche Nachricht-----
Von: Eric Blake [mailto:address@hidden 
Gesendet: Mittwoch, 19. November 2014 16:44
An: Paolo Bonzini; Prof. Dr. Michael Schefczyk; qemu-devel
Betreff: Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was 
Re: AW: Bug Repoting Directions Request)

On 11/19/2014 07:54 AM, Paolo Bonzini wrote:
> On 19/11/2014 13:07, Prof. Dr. Michael Schefczyk wrote:
>> Yes! My level of knowledge is that one uses the qcow2 format in order 
>> to be able to create live snapshots/backups. Otherwise one would tend 
>> to use the more efficient raw format. Is this not correct and did I 
>> apply the backup mechanism in the wrong way?
> 
> That's correct, but you still have to create live snapshots from 
> within QEMU.

You absolutely CANNOT modify a qcow2 file via qemu-img or any other external 
means while qemu is using the file in read-write mode, or you risk file 
corruption (which appears to be what happened to you).
Monitor commands to the qemu process are the only supported means for taking a 
live snapshot/backup of an in-use disk.

> 
> This is done with a QMP (QEMU Management Protocol) command like
> 
> { "execute": "blockdev-snapshot-internal-sync",
>                 "arguments": { "device": "ide-hd0",
>                                "name": "snapshot0" }
>    }
> 
> QMP is accessed through normal sockets, or via libvirt.
> 
> However, I'm not sure if running "qemu-img convert" on the resulting 
> snapshot is possible though, and there is no equivalent of "qemu-img 
> snapshot -d".

qemu-img snapshot -d can be achieved via the QMP monitor command 
'human-monitor-command' forwarding to the HMP 'delvm' command.  But you are 
correct that I don't know of any way to reproduce qemu-img convert on a 
read-write image.

> 
> You can instead use QEMU's support for backup, which will do what you 
> wanted directly while the VM is running.  For example:
> 
> { "execute": "drive-backup", "arguments": { "device": "ide-hd0",
>                                      "sync": "full", "format": "qcow2",
>                                      "target": "backup.img" } }
> 
> This does not even require qcow2 for the image.  The downside is that 
> you must not turn off the VM until the job has completed.

Another option is to create a live snapshot (so that your file under interest 
is now read-only with a temporary qcow2 overlay as the read-write file), then 
you can do whatever you want with the file (such as qemu-img convert), and 
finally do an active commit to get rid of the temporary qcow2 overlay and back 
to just the main live image.

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

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