UVB and Changed Block Tracking (CBT)


UVB and Changed Block Tracking (CBT)


Changed Block Tracking (CBT) reduces both the backup window and storage requirements for your backup jobs by reading and writing only the data that has changed since the previous backup. CBT is also used when performing Rollback restores, writing only the changes from the backup over the existing VM, restoring data in a fraction of the time it takes to restore the entire machine.



You can enable CBT per-VM when creating a backup job using the Protect > Backup, selecting the VM to protect, then clicking the pencil User-added image image.

User-added image


How to Reset Change Block Tracking (CBT) for VMware Backups

To reset the CBT for a VMware Guest VM, review the following How to Reset Change Block Tracking (CBT) for VMware Backups.


CBT and VMware

With VMware, the VMware's vStorage API is used to enable Changed Block Tracking to reduce the time your backups take and the amount of data sent to your backup storage location.

When CBT is enabled for a backup job, each VM in the job is checked to see if it is at least hardware version 7. If the VM meets this requirement, the CBT configuration parameter (ctkEnabled = true) is enabled for that VM the first time the job runs. If a VM is not at least hardware version 7, a warning is logged and a non-CBT backup is performed.

The initial backup that takes place with CBT enabled reads all blocks of the VM's virtual disks to create a change ID for the VM. The next time the backup job is run, the change ID is used to determine only the blocks that have changed since the last backup for each VM. Only the changed blocks are then included in the backup.

For restored, cloned, moved, or copied virtual machines, VMware cannot enable CBT until the new VM is powered on for the first time. Any backups run before the VM is powered on will be non-CBT backups.


CBT and Hyper-V

For Hyper-V, Unitrends Virtual Backup uses a custom filter driver to track changes made to virtual disks. When the UVB software takes a backup, it uses this list of changes and reads only the blocks that have changed between backups, saving a significant amount of processing time.

For additional details on current CBT support for Hyper-V, refer to the Known Issues for UVB 8.0


Note: CBT is available for VMware and Hyper-V virtual machines with UVB version 8.0 and later.

Was this article helpful?
0 out of 0 found this helpful
Have more questions? Contact us