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Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re
From: |
Prof. Dr. Michael Schefczyk |
Subject: |
Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request) |
Date: |
Wed, 19 Nov 2014 17:32:46 +0000 |
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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- [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Paolo Bonzini, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Max Reitz, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Prof. Dr. Michael Schefczyk, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Paolo Bonzini, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Prof. Dr. Michael Schefczyk, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Paolo Bonzini, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Eric Blake, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request),
Prof. Dr. Michael Schefczyk <=
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Eric Blake, 2014/11/19
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Prof. Dr. Michael Schefczyk, 2014/11/22
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Paolo Bonzini, 2014/11/24
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Kevin Wolf, 2014/11/25
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Prof. Dr. Michael Schefczyk, 2014/11/25
- Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request), Kashyap Chamarthy, 2014/11/24