Swap virtio images

Swap virtio images

[Iain J. Watson]

Hello,

I am trying to find a way to use backing files to make snapshots of
running VMs. The idea is I pause the VM, use qemu-img to create a new
disk file with the currently used file as a backing file, swap the
images then unpause the VM.

Commands would be something like:

----------
  (qemu) stop
  (qemu) drive_del virtio0

  $ qemu-img create -f qcow2 -b os-disk-0.qcow2 os-disk-1.qcow2

  (qemu) drive_add 0 file=os-disk-1.qcow2   # I'm not 100% sure on the
drive_add syntax
  (qemu) c
----------

If I try and do this just now I get the error "Can't hot-add drive to
type 7" when running the drive_add command.

Is there another way to achieve this? Is this something that could
potentially be added or would architecture not support what I'm trying
to do?

Cheers,
Iain Watson
-- 
"Just think how stupid the average person is,
and then realize that half of them are even stupider!"
--George Carlin

Re: Swap virtio images

[Mauricio Tavares]

On Wed, Jun 6, 2012 at 9:35 AM, Iain J. Watson <lists@ij-watson.co.uk> wrote:
> Hello,
>
> I am trying to find a way to use backing files to make snapshots of
> running VMs. The idea is I pause the VM, use qemu-img to create a new
> disk file with the currently used file as a backing file, swap the
> images then unpause the VM.
>
> Commands would be something like:
>
> ----------
>  (qemu) stop
>  (qemu) drive_del virtio0
>
>  $ qemu-img create -f qcow2 -b os-disk-0.qcow2 os-disk-1.qcow2
>
>  (qemu) drive_add 0 file=os-disk-1.qcow2   # I'm not 100% sure on the
> drive_add syntax
>  (qemu) c
> ----------
>
> If I try and do this just now I get the error "Can't hot-add drive to
> type 7" when running the drive_add command.
>
> Is there another way to achieve this? Is this something that could
> potentially be added or would architecture not support what I'm trying
> to do?
>
      This is probably not the answer you seek, but I create a lvm
logical volume for each vm. So, if I need to backup the "pickles" vm I
create a lvm snapshot, which can be done while the vm is happily
running along:

lvcreate -L 10G -s -n pickles_snap_`date "+%Y%m%d"` ${HOSTNAME}_vg0/pickles

If I need to go back to the snapshot I can lvconvert --merge.

> Cheers,
> Iain Watson
> --
> "Just think how stupid the average person is,
> and then realize that half of them are even stupider!"
> --George Carlin
> --
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Re: Swap virtio images

[Kevin Wolf]

Am 06.06.2012 15:35, schrieb Iain J. Watson:
> Hello,
> 
> I am trying to find a way to use backing files to make snapshots of
> running VMs. The idea is I pause the VM, use qemu-img to create a new
> disk file with the currently used file as a backing file, swap the
> images then unpause the VM.
> 
> Commands would be something like:
> 
> ----------
>   (qemu) stop
>   (qemu) drive_del virtio0
> 
>   $ qemu-img create -f qcow2 -b os-disk-0.qcow2 os-disk-1.qcow2
> 
>   (qemu) drive_add 0 file=os-disk-1.qcow2   # I'm not 100% sure on the
> drive_add syntax
>   (qemu) c
> ----------
> 
> If I try and do this just now I get the error "Can't hot-add drive to
> type 7" when running the drive_add command.
> 
> Is there another way to achieve this? Is this something that could
> potentially be added or would architecture not support what I'm trying
> to do?

You're looking for the 'snapshot_blkdev' monitor command:

(qemu) snapshot_blkdev virtio0 os-disk1.qcow2 qcow2

Kevin

Re: Swap virtio images

[Iain J. Watson]

On Wed, Jun 6, 2012 at 3:27 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 06.06.2012 15:35, schrieb Iain J. Watson:
>> Hello,
>>
>> I am trying to find a way to use backing files to make snapshots of
>> running VMs. The idea is I pause the VM, use qemu-img to create a new
>> disk file with the currently used file as a backing file, swap the
>> images then unpause the VM.
>>
>> Commands would be something like:
>>
>> ----------
>>   (qemu) stop
>>   (qemu) drive_del virtio0
>>
>>   $ qemu-img create -f qcow2 -b os-disk-0.qcow2 os-disk-1.qcow2
>>
>>   (qemu) drive_add 0 file=os-disk-1.qcow2   # I'm not 100% sure on the
>> drive_add syntax
>>   (qemu) c
>> ----------
>>
>> If I try and do this just now I get the error "Can't hot-add drive to
>> type 7" when running the drive_add command.
>>
>> Is there another way to achieve this? Is this something that could
>> potentially be added or would architecture not support what I'm trying
>> to do?
>
> You're looking for the 'snapshot_blkdev' monitor command:
>
> (qemu) snapshot_blkdev virtio0 os-disk1.qcow2 qcow2
>
> Kevin

Ha, it was right in front of my eyes all the time. I thought I'd read
that would only do the qcow2 internal snapshot. Guess I was wrong.

Quickly trying that throws up an error about not being able to open
the new file but I'll debug that a bit before seeking more help.

Cheers,
Iain
-- 
"Just think how stupid the average person is,
and then realize that half of them are even stupider!"
--George Carlin

Re: Swap virtio images

[Iain J. Watson]

On Wed, Jun 6, 2012 at 4:26 PM, Jim <jim@suresafe.co.uk> wrote:
>
>
> Is this not what virsh create-snapshot is supposed to do for you ?
>
> However, https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/1004606
> suggests this is not really working right now.
>
> Is this an Ubuntu screw-up or is it a known issue not yet selected for
> fixing (the bug is shown as confirmed) ?
>
>

For my own question, I'm deliberately not using libvirt so I can't
comment on that.

I can happily confirm that the monitor command 'snapshot_blkdev' works
fine for me on Ubuntu 12.04

Cheers,
Iain
-- 
"Just think how stupid the average person is,
and then realize that half of them are even stupider!"
--George Carlin