Status of Snapshots

[Jorge Fábregas]

Hi again,

I noticed no "snapshot" functionality in virt-manager :(  What I found
while doing a search is that it is planned but couldn't find anything
recent.  What's the current status on snapshots? Any plans to include it
with virt-manager for F15? F16?

Also,  is it to shut down the VM, and just copy the *.img files
somewhere else the closest thing we have now ? or place the *.img files
on a logical-volume and then perform the snapshot there (LVM wise)?

Thanks,
Jorge

[Richard W.M. Jones]

On Sun, Jan 09, 2011 at 05:02:50PM -0400, Jorge Fábregas wrote:
>  I noticed no "snapshot" functionality in virt-manager :( 
> What I found
>  while doing a search is that it is planned but couldn't find anything
>  recent.  What's the current status on snapshots? Any plans to include it
>  with virt-manager for F15? F16? 
This is really a qemu issue, rather than virt-manager.  Someone from
Red Hat is working on snapshot patches for qemu at the moment.  No ETA
that I know of ... I understand that it's a rather difficult problem
to solve well.

>  Also,  is it to shut down the VM, and just copy the *.img files
>  somewhere else the closest thing we have now ? or place the *.img files
>  on a logical-volume and then perform the snapshot there (LVM wise)? 
There's various different things you can do at the moment:

(1) If your block layer is able to take an instantaneous snapshot,
then you can use this to get a "crash consistent" snapshot of a live
VM.  It will require journal recovery (which you can do with
libguestfs for example), but apart from that should be usable for most
things except where you have complex guest applications like databases
which need their own consistency.

This feature is available on filers like NetApp, or on LVM using
'lvcreate -s'.  It's *not* safe eg. to 'dd' or 'cp' a disk
image,
because that is not snapshotting the whole disk at one instant.

(2) If you pause the VM, you can use a 'dd'/'cp' to get a crash
consistent snapshot.  Of course the long pause is not usually
acceptable.

(3) If you shut the VM down first, you can snapshot or copy the disk
image, and the copy you get will be consistent.  What you're really
getting here is a clone, and if you want to go all the way with
cloning there are other things you need to adjust:

https://rwmj.wordpress.com/2010/09/24/tip-my-procedure-for-cloning-a-fedo...

You should thoroughly test whatever procedure you use before you use
it for your mission-critical backups ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.
http://et.redhat.com/~rjones/virt-p2v

[Tom Horsley]

On Sun, 9 Jan 2011 23:19:49 +0000
Richard W.M. Jones wrote:

>  (3) If you shut the VM down first, you can snapshot or copy the disk
>  image, and the copy you get will be consistent. 
Or even make a new image using the old one as a backing file. I like
to do that because I can make lots of copies of the original image
that share the same disk space.

[Ján ONDREJ (SAL)]

Hello,

  there are 2 types of snapshoting in virtualization. You can snapshot whole
virtual machine (it's RAM, registers and also hard disks) and snapshots of
disks only. Disk snapshotting is well implemented in qemu/kvm, but there is
no support in libvirt. I have an very old request for enhance in bugzilla:
  https://bugzilla.redhat.com/show_bug.cgi?id=508662

Qemu disk snapshotting is a very good thing. I use it for testing purposes,
for example if I need to test something in new fedora, I use -drive ...,
snapshot=on option in qemu, I can do anything and after VM shutdown all
changes are gone. If I need to save these changes, there are abilities to
commit all changes back to original image. I can use snapshotting with
compresses images and images without full allocation.

  I think, it's not hard to implement this in libvirt, but my knowledge of
C programming is not enough to do this. Any other volunteer?

			SAL

On Sun, Jan 09, 2011 at 11:19:49PM +0000, Richard W.M. Jones wrote:
>  On Sun, Jan 09, 2011 at 05:02:50PM -0400, Jorge Fábregas wrote:
>  > I noticed no "snapshot" functionality in virt-manager :(  What I found
>  > while doing a search is that it is planned but couldn't find anything
>  > recent.  What's the current status on snapshots? Any plans to include it
>  > with virt-manager for F15? F16?
>  
>  This is really a qemu issue, rather than virt-manager.  Someone from
>  Red Hat is working on snapshot patches for qemu at the moment.  No ETA
>  that I know of ... I understand that it's a rather difficult problem
>  to solve well.
>  
>  > Also,  is it to shut down the VM, and just copy the *.img files
>  > somewhere else the closest thing we have now ? or place the *.img files
>  > on a logical-volume and then perform the snapshot there (LVM wise)?
>  
>  There's various different things you can do at the moment:
>  
>  (1) If your block layer is able to take an instantaneous snapshot,
>  then you can use this to get a "crash consistent" snapshot of a live
>  VM.  It will require journal recovery (which you can do with
>  libguestfs for example), but apart from that should be usable for most
>  things except where you have complex guest applications like databases
>  which need their own consistency.
>  
>  This feature is available on filers like NetApp, or on LVM using
>  'lvcreate -s'.  It's *not* safe eg. to 'dd' or 'cp' a disk
> image,
>  because that is not snapshotting the whole disk at one instant.
>  
>  (2) If you pause the VM, you can use a 'dd'/'cp' to get a crash
>  consistent snapshot.  Of course the long pause is not usually
>  acceptable.
>  
>  (3) If you shut the VM down first, you can snapshot or copy the disk
>  image, and the copy you get will be consistent.  What you're really
>  getting here is a clone, and if you want to go all the way with
>  cloning there are other things you need to adjust:
>  
>  https://rwmj.wordpress.com/2010/09/24/tip-my-procedure-for-cloning-a-fedo...
>  
>  You should thoroughly test whatever procedure you use before you use
>  it for your mission-critical backups ...
>  
>  Rich.
>  
>  -- 
>  Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
>  virt-p2v converts physical machines to virtual machines.  Boot with a
>  live CD or over the network (PXE) and turn machines into Xen guests.
>  http://et.redhat.com/~rjones/virt-p2v
>  _______________________________________________
>  virt mailing list
>  virt(a)lists.fedoraproject.org
>  https://admin.fedoraproject.org/mailman/listinfo/virt

[Jes Sorensen]

On 01/10/11 00:19, Richard W.M. Jones wrote:
>  On Sun, Jan 09, 2011 at 05:02:50PM -0400, Jorge Fábregas wrote:
> > I noticed no "snapshot" functionality in virt-manager :(  What I found
> > while doing a search is that it is planned but couldn't find anything
> > recent.  What's the current status on snapshots? Any plans to include it
> > with virt-manager for F15? F16?
>  
>  This is really a qemu issue, rather than virt-manager.  Someone from
>  Red Hat is working on snapshot patches for qemu at the moment.  No ETA
>  that I know of ... I understand that it's a rather difficult problem
>  to solve well. 
Actually basic snapshot support is available in upstream QEMU now. It is
yet to filter down into Fedora and other distros though, and none of the
management tools are aware of it to the best of my knowledge.

Cheers,
Jes

[Jorge Fábregas]

Thank you all for the info!