Changed Block Tracking (CBT) is a VMware feature that enables efficient backup of virtual machines by tracking changes and only backing up changes rather than complete virtual machines. CBT is used automatically for restoring a full virtual machine when the Restore in place and Unconditionally overwrite VM with the same name restore options are used, and if restored changes total 50 MB or more, providing significant performance improvements.
Since only changed blocks are restored, a CBT restore is much faster and more efficient than a classic restore. If a CBT restore cannot be performed or if the restore job is restarted, the restore reverts to a classic restore.
CBT restores are enabled by default; but you can disable them as described in Disabling Restores Using Changed Block Tracking.
To initiate a CBT restore, use the restore option to overwrite an existing VM.
Notes
-
CBT restores are supported for virtual machines using hardware version 7 or later.
-
CBT restores are not supported for attaching a disk to an existing virtual machine.
-
Only the first restore from a backup can use CBT. To perform another CBT restore, another backup must be performed first.
-
If you perform an incremental backup after performing a CBT restore, the incremental backup uses the changed block tracking IDs from the last backup that ran before the restore; the query of changed sectors fails and the CBT status is set to Failed in the CommCell Console.
You can ignore the CBT status; new changed block tracking IDs are generated and can be used for the next incremental backup job or for CBT restores.