[libvirt-users] Re: managedsave results in unexpected shutdown from inside Windows

[Nicolas Sebrecht]

The 13/03/13, Eric Blake wrote:

> You might want to look into external snapshots as a more efficient way
> of taking guest snapshots.

I have guests with raw disks due to Windows performance issues. It would
very welcome to have minimal downtime as some disks are quiet large
(terabytes) and the allowed downtime window very short. Let's try external
snapshots for guest "VM" while running:

  # cd virtuals/images
  # virsh
  virsh> snapshot-create-as VM snap1 "snap1 VM" --memspec file=VM.save,snapshot=external --diskspec vda,snapshot=external,file=VM-snap1.img
  Domain snapshot snap1 created
  virsh> exit
  # ls VM-snap1.img
  ls: cannot access VM-snap1.img: No such file or directory
  #

Ooch!
<investigating...>

  # ls /VM-snap1.img
  /VM-snap1.img
  # ls /VM.save
  /VM.save
  #

Surprising! I would have expect files to be stored in virtuals/images. This is
not the point for now, let's continue.

  # virsh snapshot-list VM
   Name                 Creation Time             State
  ------------------------------------------------------------
   snap1                2013-03-14 12:20:01 +0100 running
  #


USE CASE 1: restoring from backing file
=======================================

  # virsh shutdown VM

I can't find a snapshot-* doing what I want (snapshot-revert expects to
revert a snapshot), trying restore.

  # virsh restore /VM.save
  Domain restored from /VM.save
  # LANG=C virsh snapshot-list VM
   Name                 Creation Time             State
  ------------------------------------------------------------
   snap1                2013-03-14 12:20:01 +0100 running
  #

As we might expect, the snapshot is still there.

  # virsh snapshot-delete VM snap1
  error: Failed to delete snapshot snap1
  error: unsupported configuration: deletion of 1 external disk snapshots not supported yet
  #

A bit annoying. Now, it seems that I have to manually delete garbage. Actually,
I've tried and I had to delete /VM.save, /VM-snap1.img,
/var/lib/libvirt/qemu/snapshot/VM/snap1.xml and restart libvirt (no snapshot-refresh).


USE CASE 2: the files are saved in another place, let's merge back the changes
==============================================================================

The idea is to merge VM-snap1.img back to VM.raw with minimal downtime. I can't
find a command for that, let's try manually.

  # virsh managedsave VM
  # qemu-img commit /VM-snap1.img
  # rm /VM-snap1.img /VM.save
  # virsh start VM
  error: Failed to start domain VM
  error: cannot open file 'VM-snap1.img': No such file or directory
  # virsh edit VM
  <virsh edit VM to come back to vda -> VM.raw>
  # virsh start VM
  error: Failed to start domain VM
  error: cannot open file 'VM-snap1.img': No such file or directory
  #

Looks like the complain comes from the xml state header.

  # virsh save-image-edit /var/lib/libvirt/qemu/save/VM.save
  <virsh edit VM to come back to vda -> VM.raw>
  error: operation failed : new xml too large to fit in file
  #

Stuck. :-/

-- 
Nicolas Sebrecht