"Creating snapshot failed" error

Description

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

To view details:

In the Virtual Machines section, select the VM.
Enter Parameters → Information → Last 10 events.
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:

Connect to the node server via SSH.
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.

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

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>
```
Repeat the backup.