Having your backup jobs fail is really a frustrating experience. 99,5% of the time, i’m experiencing successful backups, either over TSM, or Veeam Backup.
However, there were more than a handful of times that the CBT (Change Block Tracking) stopped working, generally causing backup jobs to fail.
The message listed in the Action log is the following:
CBT data is invalid, failing over to legacy incremental backup. No action is required, next job run should start using CBT again.
If CBT data remains invalid follow KB1113 to perform CBT reset. Usual cause is power loss.
I’ve tried to narrow the reasons this happens, to mostly being Datastore capacity outage or the speed of snapshots being removed during the backup process.
Luckily, there is a solution and it lurks in the VMWare KB2139574 (Resetting Changed Block Tracking for VMware vSphere virtual machines) .
You can generally follow one of the two recommended actions according to VMWare:
- Power off the VM and reset CBT
- Run the provided script to reset CBT on all VMs, which uses a snapshot method to save having to power the VMs off.
Personally, although i have tried both actions, i prefer the second approach, as most of the virtual Infrastructures contain VMs that are not easy to power off, or have different maintenance windows.
In any case, after reattempting to execute the Veeam backup job, the error is no longer there, and Change Block Tracking is back in use.
