While replicating or backing up a VM from source to target/repository first run is a full replication/backup - all data is copied from the ESX(i) host to your destination.
After the full run, the next differential or Incremental will use CBT if enabled on the VM and will only replicate/backup the blocks that have changed since the last successful replication/backup.
Prior to ESX 4 and VM hadware 7 the software had to scan the vmdk disks to determine which blocks are new. This was CPU and memory + time intensive. CBT is a feature introduced in ESX4 that tracks what blocks have changed on the vmdks.
Changed Block Tracking
Changed Block Tracking reduces the time needed for incremental and differential backups. by only backing up the portions of a disk that have changed since the last backup. By determining which blocks changed within the VMDK file, vRanger only backs up the portions of a disk that have changed since the last backup. This will often result in shorter durations for backup operations, and reduce resource consumption on network and storage elements.
CBT can be activated in vSphere, where it is disabled by default, or in the vRanger GUI, by right-clicking on a VM and selecting “Enable Change Block Tracking.”
Please note, however, that utilizing CBT does impose some overhead on the associated ESX host - to the VMFS and to VMkernel memory.
Replication With Changed Block Tracking (CBT)
Replication with CBT is the recommended method of replication whenever possible.
When enabled, CBT will track the disk block changes made by the source VM. CBT, once enabled on the source host, records the blocks that have changed since the last replication pass and transfers them to the target host without scanning the VMDK. CBT is only available when replicating from ESX 4 host to ESX 4 host
Pre-Requisites:
To Enable CBT using vRanger:
