VBR uses this feature to track changes between incremental backups and make those backups faster. Instead of reading the whole vmdk, it will ask and receive only the changed blocks from the last incremental backup. How do you know CBT is enabled and used?
From VMware point of view, you will see for each vmdk a file with the vmdk_name-ctk.vmdk
From Veeam point of view, you will see in the backup job statistics next to the disk details CBT. Let's look at what actually happens in the backup job.
CBT on, active full backup, read 20 GB
CBT on, incremental backup, read 11 MB
CBT off, incremental backup, read 20 GB
This is CBT in action. The first picture is an active full, so the whole disk has been read. The second one is an incremental where only changes from last backup have been read. When CBT was disabled, the second incremental also read 20GB. Now extrapolate this to hundreds of VMs to understand the importance of CBT: less data read means not only a shorter backup window, but also less load on the production storage.
Luckily, CBT is enabled by default for all newly created backup jobs. It can be found in the vSphere integration tab, in Storage > Advanced.
CBT is not supported for physical mode RDMs and if a VM has snapshots when it's activated the first time. Sometimes CBT can get corrupted and the only way to solve it is to reset CBT. This is not easily done A new feature in VBR v10 allows to automatically reset CBT on all VMs when after an active full backup is executed. Remembering how many times I heard support guys advising a CBT reset, I think this is a cool feature to add. The CBT reset action is caught in the backup stats window
The downside is that the active full backup will take a bit longer, but you will be protected against potential CBT corruptions.