A differential export is an export of the differences of a virtual machine from between two points in time. It is used to enable an incremental backup of a virtual machine that is backed up using the new file-based backup system with Resilient Change Tracking. The below image shows the state of a VM and its backup after a full backup. Note that this file-based backup has used Resilient Change Tracking to identify what changes are being made to the VM’s storage since the backup.
An incremental backup starts, using the differential export process. A backup checkpoint, including VM configuration and VHD fork (via AVHD) is created. The existing Resilient Change Tracking ID T1 is used to determine what has changed in the parent VHD to create a differential export of the VM in the backup target media (exported VM configuration T2 and the differential VHD).
The backup checkpoint is removed and a new RCT ID (T2) is created so we can now do Resilient Change Tracking of the VHD for the time after the backup.
Old reference points (RCT IDs) can be disposed of as required.
A “synthetic full backup” process is also support for third-party backup solutions.
Hyper-V PM Taylor Brown talks about Change Tracking in his session at TechEd Europe 2014.