Changed Block Tracking (CBT) is an optional mechanism that enhances the backup performance of Azure virtual disks. When you enable CBT, the Azure API detects the changed blocks (changed page range information) on Azure virtual disks by comparing the binary large object (BLOB) snapshots. With this comparison, CBT for Azure provides better backup performance than traditional cyclic redundancy check (CRC) backups.
When you create a new subclient, CBT is enabled by default. Existing subclients continue to use the traditional CRC backup method unless you enable CBT for that subclient.
You can use Changed Block Tracking with unmanaged and managed disks.
How It Works
Azure Virtual disks are stored as page BLOBs. When you enable CBT for IntelliSnap backups, the incremental changes are only backed up during the backup copy jobs. In addition, the Azure API detects the changed blocks (changed page range information) on the Azure virtual disks by comparing the current page BLOB snapshot against the base BLOB snapshot (which was obtained during the previous CBT-enabled IntelliSnap backup/backup copy operation). Only the changed blocks that the Azure API detects are backed up and written to backup media. We will retain the BLOB snapshot even after it is used for comparison; it will be deleted based on Storage Policy retention criteria.
Note
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Changed blocks that are backed up during the backup copy jobs will only amount to only 1MB. In previous releases, the changed blocks backed up could total 4 MB. With this reduction, the system backs up less incremental data further enhancing the backup performance of Azure virtual disks. This change will only take effect after you run a full backup for the subclient where the CBT option is already enabled.
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CBT is supported only in the regions supported by Microsoft.
For more information, see Enabling or Disabling Changed Block Tracking for Microsoft Azure.