Reverting to a preexisting snapshot can cause incremental backups based on Changed Block Tracking to become inconsistent

Products

• VMware
• VMware vSphere ESXi

Issue/Introduction

Symptoms:

Reverting a snapshot for a virtual machine that has Changed Block Tracking (CBT) enabled to a snapshot older than its last incremental backup can cause inconsistencies in incremental backups of that virtual machine.

Note: Backup products that take advantage of CBT may be affected. Check with your backup vendor.

The following scenario must occur, in sequence, before there is a possibility of this issue occurring:

• A virtual machine with hardware version 7 must have a snapshot present and must have been previously backed up by a backup product leveraging CBT.
• A backup product performs an incremental backup of the virtual machine and employs CBT to determine changed blocks since the last backup.
• After the incremental backup is complete, you manually revert a snapshot on the virtual machine.
• A backup product performs an incremental backup of the virtual machine and employs CBT to determine changed blocks since the last backup.

Environment

VMware Data Recovery 1.x
VMware ESX 4.0.x
VMware ESXi 4.0.x Installable
VMware ESXi 4.0.x Embedded

Resolution

If you need to revert a snapshot of a virtual machine (that is being backed up by a CBT-based backup application) to a snapshot older than its last incremental backup, you must do a full backup of that virtual machine before using incremental backups again.

This issue is resolved in vSphere 4.1 and vSphere 4.0 Update 3. Rather than potentially providing incomplete data, a change ID obtained before the snapshot revert is now correctly considered as being invalid.

To workaround this issue in earlier vSphere releases, perform a full virtual machine backup after a snapshot revert operation. If the backup application does not allow this as an option, you need to remove the CTK files for that virtual machine. The CTK files are stored with the virtual machine on the datastore and can be removed via the Datastore Browser. This delete operation can be safely done while the virtual machine is running.