Changed Block Tracking

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    The changed block tracking (CBT) mechanism allows Veeam Backup for Microsoft Azure to perform quick and cost-efficient image-level backup to backup repositories.

    Veeam Backup for Microsoft Azure uses the CBT mechanism to reduce the amount of data read from processed virtual disks:

    • During a full backup session, Veeam Backup for Microsoft Azure reads only written data blocks, 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.

     

    Changed Block Tracking 

    When the first backup session (full backup session) is performed, Veeam Backup for Microsoft Azure creates a cloud-native snapshot of an Azure VM. Veeam Backup for Microsoft Azure sends API requests to access the content of the snapshot and detects unallocated data blocks.

    When the next incremental backup session is performed, a new cloud-native snapshot is created. Veeam Backup for Microsoft Azure sends API requests to access and compare the content of two cloud-native snapshots (a snapshot created during the previous backup session and a snapshot created during the current backup session). Snapshot comparison allows Veeam Backup for Microsoft Azure to detect data blocks that have changed since the previous backup session.

    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.