Incremental Backup

Changed Block Tracking (CBT) and Resilient Change Tracking (RCT) are VMware vSphere's and Microsoft Hyper-V's native technology that tracks the blocks of a virtual machine disk that have been changed since a certain point in time. CBT is employed on VMware vSphere virtual machines with hardware version 7 and later. RCT is employed on Microsoft Hyper-V virtual machines with configuration version 6.2 and later. It is derived from the VMware Data Protection API and Microsoft Virtual Hard Disk (VHD) API which allow 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 RCT through Microsoft Hyper-V to get a list of changed blocks since the last backup session.

With VMware CBT and Microsoft Hyper-V RCT 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.