Virtual machines perform a full backup instead of an incremental backup following a power outage or abrupt shutdown

Products

• VMware vSphere ESXi

Issue/Introduction

• After a host or virtual machine experiences an abrupt shutdown, such as a site power outage, third-party backup solutions initiate a full backup on the affected virtual machines instead of the incremental backup.
• This leads to significant storage I/O contention and performance degradation on the virtual machines.

Environment

VMware ESXi
VMware vCenter Server

Cause

During a power outage or abrupt power-off, the in-memory Changed Block Tracking (CBT) state is not properly committed and written to the datastore.

When the virtual machines are powered back on, ESXi detects this inconsistent or "dirty" state. To ensure backup integrity, ESXi automatically resets CBT for the affected VMs. When the backup solution subsequently requests CBT information for the next backup cycle, ESXi flags the previous CBT tracking data as invalid. Consequently, the backup software is forced to perform a full backup to establish a new, healthy CBT baseline.

Resolution

Virtual machines performing a full backup after an abrupt shutdown is expected behavior.

Additional Information

Administrators can proactively detect which virtual machines will require a full backup prior to the backup software initiating the job. By parsing the vmware.log file of a virtual machine immediately after it powers on following the outage, look for the exact string: "Change tracking invalid; reinitializing"

The presence of this string confirms the CBT state was reset and the next backup will be a full backup.