Snapshots on qcow2 files with Proxmox VE CBT

[gerstepe]

I have a bunch of VMs in a cluster of  Proxmox VE 8.4 Hosts i want to backup regularly. The disks of the VMs are stored as qcow2 files on a local file based storage on each host.

I created a Backup Job for some of the VMs and used the default "Change tracking" setting (Use Proxmox VE CBT).

I want to use Snapshots on these VMs.
I can create Snapshots on the VMs that are in the Backup Job, but when i try to restore one, i get an error:
 

Quote

 qemu-img: Could not apply snapshot 'test': Device has active dirty bitmaps

The VM will be stopped and locked. I have to unlock and delete the dirty bitmap included in the qcow2 files to restore the snapshot.

Am I correct in my assumption, that the use of Proxmox VE CBT Change tracking creates an dirty bitmap in the VM disks permanently, and this prohibits the use of snapshots on these VMs?

Does setting change tracking in the backup job to "Use proprietrary method" allow me to create and restore snapshots on the VMs?
 

 

[The Official Moderator]

On 10/13/2025 at 3:10 PM, gerstepe said:

I have a bunch of VMs in a cluster of  Proxmox VE 8.4 Hosts i want to backup regularly. The disks of the VMs are stored as qcow2 files on a local file based storage on each host.

I created a Backup Job for some of the VMs and used the default "Change tracking" setting (Use Proxmox VE CBT).

I want to use Snapshots on these VMs.
I can create Snapshots on the VMs that are in the Backup Job, but when i try to restore one, i get an error:
 

The VM will be stopped and locked. I have to unlock and delete the dirty bitmap included in the qcow2 files to restore the snapshot.

Am I correct in my assumption, that the use of Proxmox VE CBT Change tracking creates an dirty bitmap in the VM disks permanently, and this prohibits the use of snapshots on these VMs?

Does setting change tracking in the backup job to "Use proprietrary method" allow me to create and restore snapshots on the VMs?
 

 

Please perform the following steps:

• Open the Shell on the affected Proxmox host.

• Execute the qemu-img info command to identify the dirty bitmap name:
  qemu-img info /mnt/pve/path/to/disk.qcow2
  (/mnt/pve/path/to/disk.qcow2 – path to the affected VM and disk)
  Find the bitmap name in the output.

• Delete the bitmap using the following command:
  qemu-img bitmap --remove /mnt/pve/path/to/disk.qcow2 bitmap_name

• Perform these steps for all disks of the affected VM.

• Afterward, you can roll back to the snapshot.