Enabling Changed Block Tracking (CBT) for VMWare Guests

SummaryChanged Block Tracking (CBT) is the method VMWare uses to enable snapshots. The VMWare Host tracks all changes in the file blocks of the guest and provides differential snapshots for backup products. To take snapshots of VMWare Guests, Amanda Enterprise requires CBT to be enabled and in a good state. This article covers the Enable/Reset procedure for CBT.

RequirementsTo enable CBT the VMWare Host Server must be on VMWare Hardware Level 7 or higher and running ESXi 4 or newer.

Enabling CBT for a VMWare GuestAmanda Enterprise enables Changed Block Tracking (CBT) during a Check Host on the Backup| What page. During the Check Host, Amanda instructs the VMWare Host to enable CBT on each disk that is found in the vmx file for the selected Guest.

Reset Procedure for CBTIn cases where Changed Block Tracking (CBT) has failed, the following procedure will allow a correct reset of CBT data. Please note, it is critical to follow these steps completely and in the given order.

1. Power down the VM
2. Disable CBT:
   1. Open the vSphere Client, select the powered-off virtual machine, and click Edit… Settings > Options > Advanced/General
   2. Click the Configuration Parameters button
   3. Change values to false for the following properties:
      • ctkEnabled
      • scsi#:#.ctkEnabled (one for each virtual disk)
   4. Save the changes
3. Using the snapshot manager in the vSphere Client, delete any snapshots for the VM that start with “zmanda”
4. Power on the VM
5. Back in ZMC, select the backup entry on the Backup| What page for the VM and click the Check Hosts button at the bottom of the table. This automatically re-enables CBT for the VM.