VMware CBT is a salient and effective feature which helps to find the changed blocks of Virtual Disks for backup aspiration. Once after the Full backup is completed for a VM, it’s not necessary to backup the entire disk again. Instead the blocks that were changed after the full backup alone could be backed up, but to achieve that, the difference between disk state from the previous backup completed time upto current time must be calculated. That’s where the CBT process plays a vital role which continues to have a set of keys of the virtual disk blocks for each incremental and provide the differences between the blocks.

To the backup software developers who use VMware’s API to develop backup software for VMware environment have got prosperity in terms of producing effective results on VMware backup by using CBT. To use this fruitful process during the backup, it has to be enabled manually for the VMs. Let us discuss on how to manually Enable or Disable CBT of a VM and how Vembu is handling these as an automated process during backup.

How to Enable CBT of VM
1→ Power off the VM
2→ Right click the VM, click “Edit settings”, find the “Options” tab and click “Configuration
Parameters”
3→ Set the “ctkEnabled” value to “True”
4→ Set the “scsi0:x.ctkEnabled” value to “True” for each disk of the VM in question
5→ Open the source folder and remove any -CTK.VMDK files
6→ Power on the VM
7→ Power off the VM again. This step is needed to update the CTK table
8→ Power on the VM
9→ Rerun Backup job

If you are configuring a Virtual Machine for backup through Vembu’s VMBackup, then there is no manual effort needed to enable the CBT instead Vembu has automated the process of enabling CBT of the VM.

How to Disable CBT of VM

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1→ Power off the VM
2→ Right click the VM, click “Edit settings”, find the “Options” tab and click “Configuration
Parameters”
3→ Set the “ctkEnabled” value to “False”
4→ Set the “scsi0:x.ctkEnabled” value to “False” for each disk of the VM in question
5→ Power off the VM.

You may need to disable the CBT of a VM for some specific reasons. Once you disable the CBT for a VM which has been configured for backup through Vembu’s VMBackup, then the backup fails at the next incremental since the CBT chain is lost. Also there will be an intimation error to enable the CBT again. Once you enable CBT again, Vembu’s VMBackup will take a Full backup for that VM to avoid data loss because of CBT failure.

Go questions? Email us at: vembu-support@vembu.com for answers.

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