Changed Block Tracking

The changed block tracking (CBT) mechanism allows Veeam Plug-in for Xen to reduce the amount of data read from processed VMs, and to increase the speed and efficiency of incremental backups:

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

To detect unallocated and changed data blocks:

1. During the first (full) backup session, Veeam Plug-in for Xen creates a snapshot of a VM using native Xen capabilities. After the session completes, only the changed block tracking metadata of the snapshot is retained.
2. During subsequent sessions, new snapshots are created. The hypervisor reports the data blocks that have changed since the previous backup session, and Veeam Plug-in for Xen reads only those blocks.