Changed Block Tracking

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

• During a full backup session Veeam Backup for OLVM and RHV reads only written data blocks, while unallocated data blocks are filtered out.
• During an incremental backup session, Veeam Backup for OLVM and 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 oVirt checkpoint ID functionality:

1. During the first (full) backup session, Veeam Backup for OLVM and RHV takes a snapshot of a VM using native oVirt capabilities. Veeam Backup for OLVM and RHV sends API requests to detect unallocated data blocks and to access all written data blocks. The written data blocks are then stored in a backup repository as a single full backup file in the native Veeam format.

While processing the requests, oVirt creates a checkpoint ID for the backup session and saves the ID to the backup metadata. Checkpoint IDs allow oVirt to track data blocks that change between sessions.

2. During every subsequent session, a new snapshot is taken and a new checkpoint ID is created. Veeam Backup for OLVM and RHV sends API requests to oVirt to use the previous checkpoint ID to detect data blocks that have changed since the previous backup session. These data blocks are then stored in the backup repository as a single incremental backup file in the native Veeam format.

Limitations for Changed Block Tracking

Due to oVirt KVM technical limitations, checkpoint IDs are not created for disks in the RAW format. Therefore, Veeam Backup for OLVM and RHV will not be able to use CBT when processing VMs with RAW disks attached. If CBT cannot be used, Veeam Backup for OLVM and RHV reads the whole content of VM disks and compares it with backed-up data that already exists in backup repositories. In this case, the completion time of incremental backups may occur to grow.