This is an archive version of the document. To get the most up-to-date information, see the current version.

Changed Block Tracking

The changed block tracking (CBT) mechanism allows Veeam Backup for RHV to increase the speed and efficiency of incremental backups:

During a full backup session Veeam Backup for RHV reads only written data blocks, while unallocated data blocks are filtered out.
During an incremental backup session, Veeam Backup for RHV reads only those data blocks that have changed since the previous backup session.

To detect unallocated and changed data blocks, CBT relies on the RHV REST API:

During the first (full) backup session, Veeam Backup for RHV creates a snapshot of a VM using native RHV capabilities. To do that, Veeam Backup for RHV sends API requests to access the content of the snapshot and to detect unallocated data blocks.
During subsequent sessions, new snapshots are created. Veeam Backup for RHV sends API requests to access and to compare the content of the snapshot created during the previous backup session and the snapshot created during the current backup session. This allows Veeam Backup for RHV to detect data blocks that have changed since the previous backup session.

Important

If Veeam Backup for RHV is unable to use CBT while creating incremental backups, you may get the following warnings in backup session logs:

"Unable to enable oVirt incremental backups for disk. Full scan backups will be performed". To resolve the issue, follow the Veeam KB article.
"The Disk id=<disk id> has RAW format and can be backed up only in full scan mode". To resolve the issue, take a VM snapshot in the RHV Administration Portal as described in Red Hat Virtualization documentation. Alternatively, back up the VM and restore its VM disks with the Restore all VM disks to QCOW2 format option selected at the Configure Mapping Settings step of the Virtual Disk Restore wizard.