Re: disk image snapshot functionality

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

Hi,

(2010/09/29 5:24), Peter Doherty wrote:
> Hi,
>
> I'm using Centos5.5 on the host, and the KVM that's available in the repos. I'm using linux VMs too. My disk images are qcow2 files.
> Here's what I want:
> To be able to, on the host, create a snapshot of the guest's disk image, without shutting down the guest, so that I can then restore back to a point in time for the guest.

Yesterday, I heard a similar desire at the Virtualization End User Discussion @ LinuxCon Japan.

What you want to do is:
   - taking the whole VM disk image snapshot
     or
   - just taking specific partition's image?

Generally speaking, taking the whole VM disk image snapshot without breaking
guest side consistency seems difficult.

Even though you restrict to a specific partition, you need to guarantee that
the file system is in an appropriate state.


Anybody has better ideas?

   Takuya


> I thought I could do this with the qcow2 images.
> I've used:
> qemu-img snapshot -c snapname disk_image.qcow2
> to create the snapshot.
>
> It doesn't work. The snapshots claim to be created, but if I shut down the guest, apply the snapshot
> ( qemu-img snapshot -a snapname disk_image.qcow2 )
> the guest either:
> a.) no longer boots (No bootable disk found)
> b.) boots, but is just how it was when I shut it down (it hasn't reverted back to what it was like when the snapshot was made)
>
>
> It makes no sense. I can sometimes apply the first snapshot, and it has worked...but subsequent snapshots are a no go.
> One thing that is suspicious is that the VM SIZE and CLOCK are zero:
> # qemu-img snapshot -l test1.qcow2
> Snapshot list:
> ID TAG VM SIZE DATE VM CLOCK
> 1 with100mb 0 2010-09-28 11:48:23 00:00:00.000
> 2 with200mb 0 2010-09-28 11:50:53 00:00:00.000
> 3 with300mb 0 2010-09-28 11:52:49 00:00:00.000
> 4 whenoff 0 2010-09-28 11:56:41 00:00:00.000
>
>
> # /usr/libexec/qemu-kvm --help
> QEMU PC emulator version 0.9.1 (kvm-83-maint-snapshot-20090205), Copyright (c) 2003-2008 Fabrice Bellard
>
>
> I can't find much info about using qcow2 images when I search. Any help would be appreciated.
>
> Thanks.
>
> Peter
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