Incremental/Differential Backups of a VM on vSphere 6.5 with snapshots created prior to Changed Block Tracking (CBT) being enabled could be corrupt
Article ID: 100033481
Updated On:
Resolution
A hotfix is now available for this issue in the current version of the product mentioned in this article. Refer to the Hotfix link under Related Articles to obtain the hotfix needed to resolve the issue.
Backup Exec 16 Revision 1142 Hotfix 124683 ( 000124683)
Note:
As of Backup Exec 16 Hotfix 124683, Backup Exec detects when a VM has an existing snapshot prior to enabling CBT on a VM. When it detects this condition on a VM running on vSphere 6.5 or greater, Backup Exec will not enable CBT on the VM. This leads to inflated (but correct) incremental/differential backups.
Note that Backup Exec cannot detect if an outside entity has enabled CBT on a VM with an existing snapshot. If an outside entity enables CBT on a VM with a pre-existing snapshot running on vSphere 6.5, any incremental/differential backups of that may possibly be corrupt or incomplete.
To correct this issue follow the procedure in VMware KB article https://kb.vmware.com/kb/2090639 to reset CBT on the VM, followed by a full backup of the VM.
Backup Exec 16 Revision 1142 Hotfix 124683 ( 000124683)
Note:
As of Backup Exec 16 Hotfix 124683, Backup Exec detects when a VM has an existing snapshot prior to enabling CBT on a VM. When it detects this condition on a VM running on vSphere 6.5 or greater, Backup Exec will not enable CBT on the VM. This leads to inflated (but correct) incremental/differential backups.
Note that Backup Exec cannot detect if an outside entity has enabled CBT on a VM with an existing snapshot. If an outside entity enables CBT on a VM with a pre-existing snapshot running on vSphere 6.5, any incremental/differential backups of that may possibly be corrupt or incomplete.
To correct this issue follow the procedure in VMware KB article https://kb.vmware.com/kb/2090639 to reset CBT on the VM, followed by a full backup of the VM.
Issue/Introduction
Backup Exec uses the VMWare Changed Block Tracking (CBT) feature to produce incremental/differential backups. To do so, it calls the VMware API to enable CBT tracking prior to the first backup. It then uses the VMware QueryChangedDiskAreas API to get the list of extents that changed since a given snapshot.
VMware does not support enabling CBT on a VM that has one or more existing snapshots. See VMware KB article http://kb.vmware.com/kb/1020128 for the requirements to enable CBT. Prior to vSphere 6.5, the QueryChangedDiskAreas API would return an error if the CBT data was corrupt. This would lead to an inflated (but correct) incremental/differential backup. However, as of vSphere 6.5, the QueryChangedDiskAreas API no longer returns an error if the CBT data is corrupt. This leads to a possibly corrupt or incomplete incremental/differential backup.
Versions of Backup Exec prior to Backup Exec 16 Hotfix 124683 do not detect that a VM has an existing snapshot prior to enabling CBT on a VM. This can result in possibly corrupt incremental/differential backups due to the change in the behavior of the QueryChangedDiskAreas API.
VMware does not support enabling CBT on a VM that has one or more existing snapshots. See VMware KB article http://kb.vmware.com/kb/1020128 for the requirements to enable CBT. Prior to vSphere 6.5, the QueryChangedDiskAreas API would return an error if the CBT data was corrupt. This would lead to an inflated (but correct) incremental/differential backup. However, as of vSphere 6.5, the QueryChangedDiskAreas API no longer returns an error if the CBT data is corrupt. This leads to a possibly corrupt or incomplete incremental/differential backup.
Versions of Backup Exec prior to Backup Exec 16 Hotfix 124683 do not detect that a VM has an existing snapshot prior to enabling CBT on a VM. This can result in possibly corrupt incremental/differential backups due to the change in the behavior of the QueryChangedDiskAreas API.
Additional Information
ETrack: 3906448
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