Changed Block Tracking Was Not Applied (Code 1934)

Situation

During an incremental backup of a Hyper-V virtual machine, the backup service could not retrieve or verify the reference point created during the previous backup. As a result, Changed Block Tracking (CBT) could not be used for this backup.

A backup plan run finishes with the following info message: Changed Block Tracking was not applied. Could not get the previous backup tracking point(s) for some VMs. The affected virtual machines are listed in the warning.

Cause

Changed block tracking (Resilient Change Tracking in Hyper-V terminology) is a feature that implements changed block tracking for Hyper-V VM disks. Changed Block tracking can be enabled by users to optimize the backup procedure and save backup data reading time. Once the Changed Block Tracking option is enabled, only changed blocks of a virtual machine disk (vhdx) between two backup operations are read.

This issue occurs when a reference point required for Changed Block Tracking was unavailable or could not be verified.

Why the reference point may not be found

The reference point may be unavailable for several reasons:

• VM checkpoints were manually deleted or removed by third-party software.
• The VM was migrated between Hyper-V hosts.
• The VM was exported and then imported.

Solution

No action is normally required. The backup continues without Changed Block Tracking.

For the affected VM, the current incremental backup reads the virtual disks and compares their contents instead of using the Changed Block Tracking map to identify changed blocks. As a result, the backup may take longer than usual, but the backup itself completes successfully.

The current backup creates a new reference point. If it is available during the next backup, Changed Block Tracking is automatically will be applied again.

When to investigate

This warning is usually self-resolving if it occurs only once.

Investigate the issue if the info appears during every or most backup runs for the same VM. Repeated failures to obtain the previous reference point may indicate that something is systematically removing or invalidating the reference points between backups.