Changed Block Tracking (CBT) is a VMware feature that tracks changed disk sectors and improves the performance of VM backups. By default, the vSphere Agent enables Changed Block Tracking (CBT) for VMs.
However, because CBT requires some virtual disk processing overhead, you can stop the Agent from enabling CBT for VMs. This does not disable CBT for VMs that already have it enabled through the Agent or another mechanism. It only stops the Agent from enabling CBT in the future for VMs that do not already have it enabled.
To change the CBT setting for a vSphere Recovery Agent:
1. In Portal, on the navigation bar, click Computers.
The Computers page shows registered computers.
2. Find the vSphere Recovery Agent, and expand its view by clicking its row.
3. On the vCenter Settings tab, do one of the following:
•To enable CBT for VMs that do not have it enabled, select Enable Change Block Tracking (CBT) for Virtual Machines during backup.
•To stop the VRA from enabling CBT for VMs, clear Enable Change Block Tracking (CBT) for Virtual Machines during backup.
4. Click Save.