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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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".

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.

Paolo