Challenge
When VMware backup is scheduled, you will get the following error message :
Failed to perform pre-backup steps :: CBT not enabled. Backup cannot continue
Cause
This issue appears if Enabling Changed Block Tracking process fails during backup. This is a known issue in VMware ESXi 5.5.x, VMware vSphere Client 5.5 and the VMware vSphere Web Client 5.5.x. .
Solution
There is no proper fix provided from VMware for this issue. However, we can modify the virtual machine’s vmx file and include the ctkEnabled flag as given below.
ctkEnabled = "true"
After adding the above attribute in the vmx file, we can start backing up the VM without any issues.
Source : VMware knowledge base
Note : This issue won't occur in case if the ESXi server 5.5.x is added in the vCenter version 6.0 and backup configured by adding vCenter in vembu instance.
