Source:Changed Block Tracking (CBT)

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Overview

SEP sesam provides efficient data protection for virtual machines (VMs) running on VMware ESXi servers by using VMware vSphere Storage APIs – Data Protection (VADP). Part of the VADP is also Changed Block Tracking (CBT).

The CBT feature is an incremental backup technology from VMware that identifies changes on block level and backs up only disk blocks of a VM’s virtual disk that have changed since the last backup. SEP sesam uses CBT to perform block level incremental (INCR) and differential (DIFF) backups of VMDKs after a full (FULL) backup has been successfully executed.

SEP sesam enables CBT automatically by default to optimize backups of VMs. This way the amount of copied data is reduced, which results in reduced amount of time for the backups, reduced network bandwidth required for transferring data, and reduced required storage capacity.

CBT tracks changes of the corresponding VMDK file in a special Changed Block Tracking log file. This CTK file is permanent and must not be deleted after the backups.

Prerequisites for using CBT

SEP sesam enables CBT globally by default if the following prerequisites are met:

  • ESX/ESXi hosts at version 4.0 or newer
  • VMs must be using version 7 virtual hardware or newer
  • I/O operations must go through the ESX/ESXi storage stack. NFS is supported, and RDM is supported in virtual compatibility mode, but not in physical compatibility mode. VMFS is supported, whether backed by SAN, iSCSI, or local disk.
  • Virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.
  • If there are any existing snapshots of VM, CBT cannot be enabled.

For more details on CBT, see the VMware article Changed Block Tracking (CBT) on virtual machines.

Disabling and enabling CBT

By default, SEP sesam enables the CBT functionality globally. CBT is enabled per virtual disk and can also be disabled and enabled in the backup task properties.

Information sign.png Note
Typically, there is no need to disable CBT. By disabling CBT, the respective VM's FULL/DIFF/INC backup chain is aborted without the possibility to perform consecutive DIFF and INC backups. Only COPY backups are possible which means that the whole VMDK including empty blocks (provisioned size means that the size of a backup and the size of VMDK equals) will be backed up each time.
  1. In the SEP sesam GUI, from the menu bar select Configuration -> Defaults, and the switch to tab Extras.
  2. Uder the Extras options, select or deselect the check box Globally activate Change Block Tracking (CBT).
  3. File:CBT global.png

See also

See also VMware requirements & restrictionsVMware BackupVMware RestoreVMware Single File RestoreVMware Instant Recovery