"Creating snapshot failed" error

Description

An error appears in the interface when attempting to create a VM backup.

To view details:

1. In the Virtual Machines section, select the VM.
2. Enter Parameters → Information → Last 10 events.
3. Click on the Failed status in the Event column. This displays the Task Information tab with the detailed error text:

Unhandled exception: Error message: Creating snapshot fail
Command exited with message error: unsupported configuration: nothing selected for snapshot 

The error occurs if the VM snapshot already exists.

Solution

To resolve the issue:

1. Connect to the node server via SSH.

2. Get a list of virtual disks:

   virsh domblklist <libvirt_domain>

   Comment

   <libvirt_domain> — VM internal name in the format <VM id>_<VM name>. For example, 2698_test-alma8

   Example output:

   # virsh domblklist 2698_test-alma8
   Target Source
   ------------------------------------------------
   vda /vm2/2711_test-alma8.2698_test-alma8_snapshot

   The output shows that the VM already has a snapshot.

3. Compare the modification dates of the virtual disk master file and the snapshot. Determine which file was modified later:

   stat <virtual_disk_file>
   stat <snapshot_file>

   Example output for the virtual disk master file. See Modify value:

   # stat /vm2/2711_test-alma8
    File: /vm2/2711_test-alma8
    Size: 2419458048 Blocks: 4725400 IO Block: 4096 regular file
   Device: 902h/2306d Inode: 23330831 Links: 1
   Access: (0644/-rw-r--r--) Uid: ( 107/ qemu) Gid: ( 107/ qemu)
   Access: 2023-11-20 12:04:25.826960539 +0200
   Modify: 2023-11-20 12:06:15.420142110 +0200
   Change: 2023-11-20 12:06:15.420142110 +0200
    Birth: 2023-10-26 08:38:45.574566975 +0300

4. The next steps depend on which file was modified later and is relevant:
   

   • if the disk file is more relevant, delete the snapshot:

     rm <snapshot_file>

   • If the more relevant file is the snapshot file:

     1. Merge the file data:

        virsh blockcommit <libvirt_domain> vda --active --verbose --pivot

        Comment

        <libvirt_domain> — VM internal name in the format <VM id>_<VM name>. For example, 2698_test-alma8

        Example reply for a successful merge:

        # virsh blockcommit 2698_test-alma8 vda --active --verbose --pivot
        Block commit: [100 %]
        Successfully pivoted

     2. Delete the snapshot:

        rm <snapshot_file>

5. Repeat the backup.