Backup considerations
Changed block tracking (CBT)
Changed block tracking (CBT) is a feature that is supported for VMware vCenter Server backups. When CBT is enabled, the DS-Client attempts to reconfigure virtual machines to allow tracking of changed blocks of data. This can improve the speed of incremental backup activities, but can also slow down the virtual machine since it consumes additional resources.
When CBT is enabled, the first backup activity scans the virtual machine as follows:
• For thick disks, it scans the entire virtual machine disk.
• For thin disks, it scans only the space occupied by the disk.
Subsequent backup activities track the changes for the virtual machine files and only retrieve the blocks of data that have changed. These blocks are processed for master/delta generations and then sent to DS-System and/or local storage. If CBT fails or is not enabled, the DS-Client continues the incremental backup by reading all the data from the virtual machine disk.