Changed Block Tracking (CBT) is a mechanism that enhances the backup performance of Azure Stack Hub virtual disks. With CBT, Azure Stack Hub API detects the changed blocks (changed page range information) on Azure Stack Hub virtual disks by comparing the binary large object (BLOB) snapshots. With this comparison, CBT for Azure Stack Hub provides better backup performance than traditional cyclic redundancy check (CRC) backups.
You can use Changed Block Tracking with Azure Stack Hub managed disks only.
Note
CBT is enabled by default for newly created VSA Azure stack HUB client and VM groups. Existing subclients use the traditional CRC backup method unless you enable CBT for that subclient.
How It Works
Azure Stack Hub virtual disks are stored as page BLOBs. With CBT enabled, the Azure Stack Hub API detects the changed blocks (changed page range information) on the Azure Stack Hub 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 Stack Hub 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
Changed blocks that are backed up during the backup copy jobs will amount to only 1MB. The system backing up only 1MB of incremental data further enhances the backup performance of Azure Stack Hub virtual disks.