Virtual machine backups with resilient change tracking (RCT)
For Microsoft Windows Server 2016 or later versions, the resilient change tracking (RCT) feature is used to back up virtual machines (VMs).
RCT is a feature that provides built-in change block tracking capability for Hyper-V VM disks. Data Protection for Microsoft Hyper-V uses RCT to track changes to a VM disk (VHDX) that occur in between backup operations. The changes are tracked at the data block level. Only blocks that have changed since the last backup operation are candidates for the next incremental-forever incremental backup.
Windows Server 2016 also provides the capability to create backup snapshots (also known as checkpoints) directly without using Microsoft Volume Shadow Copy Service (VSS) (although VSS is still used inside Windows guest VMs to quiesce the VMs for application-consistent backups).
You can group several VMs in a single snapshot. However, if a guest VM that hosts applications is enabled for application protection, the VM snapshot is taken individually. For more information about application protection, see Protecting in-guest applications.
VM backup operations with RCT require the Hyper-V VM to be Version 6.2 or later.
If your VM was created on the Windows Server 2012 R2 or earlier operating system, and then later moved to a Windows Server 2016 host server (or the host server was upgraded to Windows Server 2016), you must take the VM offline and upgrade the VM version before it can be backed up. You can upgrade the VM Version by using the Hyper-V Manager or the Update-VMVersion cmdlet.
Data Protection for Microsoft Hyper-V V8.1.0 uses VSS to back up VMs in the Hyper-V environment on Windows Server 2016. Starting in V8.1.2, all Hyper-V VM backup operations in the Windows Server 2016 or later environment uses RCT. If you are upgrading from V8.1.0, because previous VSS backups do not have RCT change-tracking information, the first time you use Data Protection for Microsoft Hyper-V V8.1.7 to back up your VMs on Windows Server 2016, an incremental-forever full backup is created.
After you backed up a VM by using RCT, you can no longer use Data Protection for Microsoft Hyper-V V8.1.0 to run VSS backups on that VM.
How snapshots work with RCT backups
During an incremental-forever full backup operation for a VM, a snapshot is created of the VM disk and the snapshot contents are backed up to the IBM Spectrum Protect™ server. The snapshot is deleted automatically after the backup operation is completed.
During the next incremental-forever incremental backup, a new snapshot is created and verified against the RCT change-tracking information from the previous backup operation to determine the data that has changed. Only the changed blocks are backed up to the IBM Spectrum Protect server.
After the backup operation, the snapshot is merged with the VM by Hyper-V, and the snapshot differencing file (.avhdx) is deleted automatically. This process is unlike the VSS snapshot processing on Windows Server 2012 and 2012 R2 operating systems, in which the snapshot differencing file is retained on the VM to store incremental changes.
Any snapshot that you create manually or with another backup product do not affect the backup chain that is created by the RCT process. You can create snapshots manually or with a third-party backup product before or after a Data Protection for Microsoft Hyper-V RCT backup operation, and the next incremental backup operation by Data Protection for Microsoft Hyper-V will be based on the RCT change-tracking information from the previous backup operation.
Features that are available for RCT backups
Most Data Protection for Microsoft Hyper-V features that work on Windows Server 2012 and 2012 R2 also apply to Windows Server 2016.
However, snapshot operations are different between VSS and RCT backups. For more information, see How snapshots work with RCT backups.
Support for host failover with Cluster Shared Volumes (CSVs) is unchanged from V8.1.0 and earlier, but running a VM backup during a rolling upgrade of a Hyper-V cluster operating system is not supported.
How to query RCT backups
You can use the query VM command to display information about a VM that was backed up to the IBM Spectrum Protect server. Use the -detail parameter with the query vm command to show detailed information about the backup operation. For more information, see Query VM.
You can also use the backup vm -preview command to display the VM disk locations that can be used for the backup vm command. For more information, see Backup VM.