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Veeam Agent for Microsoft Windows 2.0
User Guide

Change Block Tracking

To perform incremental backup, Veeam Agent for Microsoft Windows needs to know what data blocks have changed since the previous job session. To get the list of changed data blocks, Veeam Agent for Microsoft Windows uses the changed block tracking mechanism, or CBT. CBT increases the speed and efficiency of incremental backups.

Change Block Tracking Note:

Veeam Agent for Microsoft Windows uses CBT for NTFS volumes only. As a result, for volumes that use other file systems, incremental backup will require greater time, because Veeam Agent for Microsoft Windows will read all data from the VSS snapshot to detect what blocks have changed since the last job session.

Veeam Agent for Microsoft Windows performs changed block tracking in the following way:

  1. During the full backup job session, Veeam Agent for Microsoft Windows reads the Master File Table (MFT) of the backed-up volume. Veeam Agent for Microsoft Windows uses MFT records to create digests with file system metadata, transfers the created digests to the target location and stores them to the resulting backup file.
  2. During subsequent incremental job sessions, Veeam Agent for Microsoft Windows performs the following operations:
  1. Reads the Master File Table (MFT) of the backed-up volume and creates the new digests with file system metadata.
  2. Interacts with the target backup location to obtain digests from the backup file that was created during the previous job session.
  3. Compares new and previous digests to detect files whose data blocks have changed on the volume since the previous job session.

During incremental backup, Veeam Agent for Microsoft Windows reads from the VSS snapshot only data blocks pertaining to files that have changed since the previous job session. If Veeam Agent for Microsoft Windows cannot calculate information about the changed files, for example, if it fails to retrieve digests from the backup file, Veeam Agent for Microsoft Windows will need to read all data blocks from the VSS snapshot. As a result, the backup window may increase significantly.

Change Block Tracking 

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