Changed Block Tracking

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    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.

    Changed Block Tracking 

    Important

    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 first cloud-native snapshot has not been removed from the chain by the retention policy before an incremental backup session runs. For more information on configuring snapshot retention settings, see Creating Backup Policies.