Article ID: 82814
Article Type: Troubleshooting
Last Modified:
If CBT is disabled or not functioning properly, the size of incremental backups can be larger than expected. In some cases issues with CBT can cause a full backup to take longer and, when restoring a lazy zero thick provisioned disk, might result in the disk becoming eager zero thick provisioned.
You can verify CBT issues in the CommCell Console by checking the Status Details dialog for a virtual machine backup job. The CBT Status field shows Disabled.
To display the Status Details dialog, right-click a backup job and select View Job Details, then click the Virtual Machine Status tab and double-click a virtual machine entry.
The vsbkp.log file includes information such as the following:
4168 6 08/15 13:37:42 98795 ### QueryChangedDiskAreas --- Exception during QueryChangedDiskAreas Error caused by file /vmfs/volumes/50f432d3-ba380107-2b26-90b11c070562/VM/VM-000003.vmdk
Note:Backups for ESXi 4.x or ESXi 5.x can also increase because of a VMware error as described inIncrease in Backup Time after Fix for VMware CBT Error for expanded VMDKs.
Changed Block Tracking (CBT) is a VMware feature that identifies blocks of data that have changed or are in use. It enables incremental backups to identify changes from the last previous backup, writing only changed or in-use blocks. If CBT is disabled or not functioning properly, an incremental backup might back up a complete virtual machine disk instead of only changed blocks, or revert to a full backup.
You can disable CBT to restart change tracking for subsequent backups. This resolution is applicable for ESX hosts 4.0 or later, and for virtual machines at hardware version 7 and later. Before resetting CBT, ensure that there are no snapshots on the virtual machine.
To reset CBT for a specific virtual machine, you must disable CBT, power cycle the virtual machine, and run a new backup. When the new backup is run, CBT is automatically re-enabled.
You can also reset CBT on multiple virtual machines using a PowerCLI script that resets CBT without requiring that virtual machines be powered down.
See the following articles to find detailed instructions for resetting CBT:
As a result of resetting CBT, the time required for backups increases because the first backup after resetting CBT backs up all blocks for any virtual machines on which CBT was reset. This change can also increase disk storage requirements for backups. The change does not affect scheduled backup cycles or data aging.