## Changed Block Tracking

Changed Block Tracking (CBT) is VMware’s native technology that tracks the blocks of a virtual machine disk that have been changed since a certain point in time. This native feature is employed on VMware VMs with hardware version 7 and later. It is derived from the VMware Data Protection API which allows third-party backup applications to take advantage of CBT to perform incremental backups. Instead of performing the full backup, Synology Active Backup for Business VM backup queries CBT through VMware and gets a list of changed blocks since the last backup session.

With VMware CBT enabled, the amount of data transferred after the first full backup will be greatly reduced and therefore speed up the backup process.

In some situations,VMware vSphere CBT might be disabled, such as when VMs run an earlier version of virtual hardware, or when CBT is disabled at the ESX host level due to free license. If Synology Active Backup for Business VM backup cannot use VMware vSphere CBT, full backup will be performed automatically.

To enable CBT for a virtual machine, please refer to this article for more information.