Create snapshot in virsh for domain with two or more disks

Question

I would like to back up the disks of a VM following the Live-disk-backup-with-active-blockcommit procedure at Libvirt Wiki.

1. I create an external snapshot with

   virsh snapshot-create-as --domain vm_name snap_name \
       --diskspec vda,file=snapshot_folder/snap.qcow2 \
       --disk-only --atomic --quiesce
   

2. Copy the base disk to the remote location.

3. Commit the disk with

   virsh blockcommit vm_name vda --active --verbose --pivot
   

4. Delete the snapshot with

   virsh snapshot-delete vm_name --metadata snap_name
   

This procedure seems to work, but only with a VM with a single disk. If I try to create a snapshot for machines with two disks, the procedure fails. For instance: if a VM has two disks, vba and vdb, and I try to run the exact same command as shown in step 1 above, the command returns

error: internal error: unable to execute QEMU command 'transaction': Could not create file: Permission denied

I tried with two --diskspec parameters, one for vda and one for vdb, but the result is the same.

How can I get this to work?

Answer 1

I had the exact same problem, which I tried to share with additional information about the symptom. But my contribution was deleted by the moderator. Anyway, I have since found the problem. The version of qemu-kvm that was included in the Ubuntu packages for 18.04 is 2.11 which is really old and apparently did not have the ability to handle multiple disks in this manner. I did a wipe and reload to 20.04 which includes qemu-kvm version 4.2. The symptom is no more.

• Non-working: https://packages.ubuntu.com/bionic/qemu-kvm
• Working: https://packages.ubuntu.com/focal/qemu-kvm

Answer 2

The only solution I found so far is creating a --diskspec for every disk, but the disk to not be handled must contain the snapshot=no.

Example:

virsh snapshot-create-as --domain ${VM} 
--name "backup-${VM}" 
--atomic 
--disk-only 
--no-metadata 
--diskspec vda,file=${SNAP}/backup-snapshot-${VM} 
--diskspec vdb,snapshot=no

Note:

${VM} is the vm-name.

${SNAP} is the directory where snapshot has to be created.