CBT on Virtual Machines

What Is VMware Changed Block Tracking and How It Works

  • The backup of the virtual machine fails. You see this error similar to:
  • The backup job show errors similar to:
  • The VMware API calls are used by the backup appliance to request that a snapshot be made. The snapshot is then taken by VMware and shown to the user so that a backup may be created.
  • CBT (Changed Block Tracking) is a feature that recognizes data blocks that have changed or are currently in use. It allows incremental backups to detect changes from the previous backup and only write altered or in-use blocks.
  • The host must be ESXi 4.0 or later.
  • The virtual machine owning the disks to be tracked must be hardware version 7 or later.
  • I/O operations must go through the ESXi storage stack. NFS is supported, RDM in virtual compatibility mode, VMFS is supported, whether backed by SAN, iSCSI, or local disk.
  • CBT must be enabled for the virtual machine (see below & by default, it is enabled).
  • Virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.
  • Consolidating/Committing snapshots in VMware ESXi (1002310)
  • “A specified parameter was not correct: vm.ctkEnabled” error when cloning a virtual machine (76280)




Technical support engineer at Tekexpert

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IRAKOZE Erwan Yves

IRAKOZE Erwan Yves

Technical support engineer at Tekexpert

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