Changed Block Tracking (CBT) is a mechanism that enhances the backup performance of Azure virtual disks. It is enabled by default. With CBT, 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 will continue to use the traditional CRC backup method unless you enable CBT for that subclient. If you attempt to perform an incremental backup using CBT on a managed or unmanaged VM, Commvault will use CRC instead.
You can use Changed Block Tracking with unmanaged and managed disks.
How It Works
Azure virtual disks are stored as page BLOBs. With CBT enabled, 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 backup operation). Only the changed blocks that the Azure API detects are backed up and written to backup media. After the incremental backup operation is complete, the base BLOB snapshot that was used for comparison is deleted. The current operation's snapshot is used as the base BLOB snapshot for the next CBT-enabled incremental backup operation.
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.
Related Topics
Enabling or Disabling Changed Block Tracking for Microsoft Azure.