Changed Block Tracking

The changed block tracking (CBT) mechanism allows Veeam Backup for Microsoft Azure to reduce the amount of data read from processed virtual disks, and to increase the speed and efficiency of incremental backups:

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

To detect unallocated and changed data blocks, CBT relies on Azure Compute APIs.

• During the first (full) backup session, Veeam Backup for Microsoft Azure creates a cloud-native snapshot of an Azure VM. To do that, Veeam Backup for Microsoft Azure sends API requests to access the content of the snapshot and to detect unallocated data blocks.
• During subsequent sessions, new cloud-native snapshots are created. Veeam Backup for Microsoft Azure 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 Microsoft Azure to detect data blocks that have changed since the previous backup session.

To allow the CBT mechanism to be used when processing Azure VM data by a backup policy, the number of snapshots to keep in a snapshot chain must be enough to ensure that the cloud-native snapshot created during the previous backup session has not been removed from the chain by the retention policy before the next backup session runs. For more information on configuring snapshot retention settings, see Creating Backup Policies.

Consider the following example. You want a backup policy to daily create both image-level backups and cloud-native snapshots: cloud-native snapshots must be created at 7:00 AM, 9:00 AM, 11:00 AM 1:00 PM, 3:00 PM and 5:00 PM; image-level backups must be created at 7:00 AM and 5:00 PM. In this case, you must set the Snapshots to keep value to minimum 5. Veeam Backup for Microsoft Azure will run the backup policy the following way:

1. At 7:00 AM, a backup session will create a cloud-native snapshot, and then use this snapshot to create an image-level backup.
2. From 9:00 AM to 3:00 PM, backup sessions will create only cloud-native snapshots.
3. After a backup session runs at 5:00 PM, the first cloud-native snapshot (created at 7:00 AM) will still be present in the snapshot chain until the next backup session.