Changed Block Tracking

To perform incremental backup, Veeam Backup & Replication needs to know which data blocks have changed since the previous job run.

For VMware VMs with hardware version 7 or later, Veeam Backup & Replication employs VMware vSphere Changed Block Tracking (CBT) — a native VMware feature. Instead of scanning VMFS, Veeam Backup & Replication queries CBT on vSphere through VADP and gets the list of blocks that have changed since the last run of this particular job. Use of CBT increases the speed and efficiency of block‑level incremental backups. CBT is enabled by default; if necessary, you can disable it in the settings of a specific backup job.

In some situations, Veeam Backup & Replication cannot leverage VMware vSphere CBT due to VMware limitations. Whenever Veeam Backup & Replication cannot leverage VMware vSphere CBT (for example, if your VMs run an earlier version of virtual hardware or CBT is disabled at the ESX host level), it fails over to Veeam’s proprietary filtering mechanism. Instead of tracking changed blocks of data, Veeam Backup & Replication filters out unchanged data blocks. During backup, Veeam Backup & Replication consolidates virtual disk content, scans through the VM image and calculates a checksum for every data block. Checksums are stored as metadata to backup files next to VM data. When incremental backup is run, Veeam Backup & Replication opens all backup files in the chain of previous full and incremental backups, reads metadata from these files and compares it with checksums calculated for a VM in its current state. If a match is found (which means the block already exists in the backup), the corresponding block is filtered out.