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 the target 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 target backup repository as a single incremental backup file in the native Veeam format.

Changed Block Tracking Issues

Due to technical limitations, oVirt KVM does not create checkpoint IDs for sessions processing VMs with RAW disks attached, and Veeam Backup for OLVM and RHV cannot use CBT when running backup policies that protect these VMs. In this case, 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 — as a result, the completion time of incremental backup sessions may occur to grow.

Eliminating Warnings

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

• "Unable to backup disks incrementally, using full scan backup" — to resolve the issue, check the oVirt configuration as described in this 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, convert the disk format to QCOW2 as described in this Veeam KB article.