Changed Block Tracking for OpenStack

Commvault Changed Block Tracking (CBT) for OpenStack is a mechanism that enhances the incremental backup performance of large OpenStack cinder volumes. When enabled, CBT generates and maintains a bitmap file (which contains snapshots) of a cinder volume before and after an incremental backup operation runs. These snapshots are used as reference points. When a CBT-enabled backup is initiated, the OpenStack cvcompute API detects the changed data blocks since the last bitmap (reference point) generated during the previous CBT-enabled backup operation for that volume. Only the changed blocks that the OpenStack cvcompute API detects are backed up and written to backup media. After the incremental backup operation is complete, the previous bitmap file (reference point) that was used is deleted. The current operation's bitmap file is then used as the reference point for the next CBT-enabled incremental backup operation.

How It Works

CBT for OpenStack utilizes Commvault's native service named cvcompute. The service, which communicates by way of the VSA using cvcompute-api endpoints, runs on all of the Nova controller nodes. These nodes communicate, using Remote Procedure Call (RPC) protocol, to a daemon cvcompute-svc running on each Nova compute node. After enabling CBT for OpenStack, when the first backup runs on a CBT-enabled subclient, the cvcompute-svc tracks the changed data blocks on each volume attached to given virtual machines. During subsequent incremental backups, when queried by VSA, the cvcompute-svc identifies the changed blocks and only those blocks will be backed up.


  • CBT is supported for OpenStack environments deployed with the KVM hypervisor.
  • CBT is supported for OpenStack streaming backups only.
  • CBT is supported for OpenStack instances created from cinder volumes only. For OpenStack instances created from glance images, CBT is disabled.
  • If a CBT-enabled backup operation fails for any reason, a new reference point is not created, and the next CBT-enabled backup runs from the last reference point.
  • If an instance is in a POWER OFF state, then CBT is disabled, and backup operations are full backups.

For more information, see Enable or Disable Changed Block Tracking for OpenStack.

Last modified: 9/10/2020 9:21:20 PM