What Exactly is CBT (Change Block Tracking)?

On a virtual machine (VM) that runs on a ESX/ESXi host, you sometimes want to track changes in disk sectors. This is where CBT comes into the picture. This VMware feature can identify disk sectors that have differences between set IDs. In addition, CBT shows which disk sectors are being employed on VMFS partitions.

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How Can It Help Me Perform a Backup?

When you complete a software backup, CBT can ask for the transmission of just the blocks on the virtual disk that have been altered since the previous backup, or only the blocks being employed. Conveniently, CBT can be used on third-party applications in connection to vSphere APIs for Data Protection. How it works is that applications call vSphere APIs for Data Protection to ask that VMkernal gives back blocks of data that have been altered on a virtual disk from the time of the previous backup snapshot.

Though CBT is incremental, it is disabled by default. Usually, only administrators can enable CBT, though some applications can enable it automatically. When CBT notices the changes since the last backup, it applies tags to the alterations and creates a CTK file, delineating all the tags. It avoids copying the VM by telling vSphere or third-party backup programs to copy the tagged blocks.


One should be aware, however, that these incremental alterations can be lost with a power failure or total shutdown. It may be better to use the full backup option more often if you have problems with power failure in your area. One also has to be aware that if one runs a VM on an earlier version than VMware 7, CBT can be disabled at the ESX host level.


CBT is an indispensable tool for the backup of disk sectors in VMs. Not only does it tag sections that have been altered from the previous backup, but it highlights which disk sectors are in use. After one has seen these reports, one can request back the data in an efficient manner.

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