If you encounter slow incremental backups when using Changed Block Tracking (CBT), you may need to reset CBT.
You run a full backup of your virtual machines (VM) running on VMware ESX/ESXi, and then run the backup a second time with only a nominal difference in completion time, and the data size during incremental backup is small and corresponds to the normal size of an incremental backup.
For example, 2TB of data takes ten hours to back up, an incremental backup takes a similar amount of time to complete, and the increase in data size is minimal and corresponds to the actual data change rate.
CBT is improperly detecting changed disk sectors on the backed up VMs, and returns the range equal to the original data backup size. Barracuda Backup must then read through and analyze all reported sector ranges. This results in the long incremental backup time.
To resolve this issue, you must manually disable and then re-enable CBT. For more information, refer to the VMware Knowledgebase solution Enabling Changed Block Tracking (CBT) on virtual machines.