Page tree

Traditionally, when a backup cycle kicks off, a full backup of a VM is performed. This process is quite complex and time-consuming. 

With the emergence of VMware Changed Block Tracking (CBT) and later Hyper-V Resilient Change Tracking (RCT), regular full backups are no longer needed. 

VMware CBT is an integral part of the latest backup technologies, providing fast and efficient backups and replication of virtual infrastructures. The CBT log file tracks changes which significantly reduces the amount of time necessary for performing incremental backups. This means that only the blocks that have been changed will undergo a backup or replication process.

Resilient Change Tracking, a built-in technology, essentially performs the same actions as CBT. Having no native CBT, the Microsoft company added the RCT feature to Windows Server 2016. This way, the users of Hyper-V 2016 and higher versions can also enjoy the benefits of block change tracking. For Nutanix AHV, this is known as Changed Regions Tracking (CRT) technology.

NAKIVO Backup & Replication uses change tracking technology to track changed data blocks and store the blocks in a Backup Repository according to your preferred retention policy (up to 1000 recovery points saved and rotated on a daily, weekly, monthly, and yearly basis). This technology allows you to recover a VM within minutes because the product does not need to go through all increments in the process of recovery. On the contrary, it only needs to pull the necessary data blocks.


  • NAKIVO proprietary change tracking technology is more time consuming than CBT/RCT/CRT and consumes resources of the VM where the product is deployed.

  • You can also enable additional checking of VMware Changed Block Tracking against NAKIVO proprietary change tracking if needed. This approach is more reliable, but may be slower than ordinary VMware CBT.

  • Licencing for certain ESXi hosts is provided in a way that disables VMware CBT by default. To enable VMware CBT manually, refer to a corresponding Knowledge Base article.

What to Consider Before Enabling CBT/RCT/CRT

If you have a VMware environment, make sure the following requirements are met to enable CBT for backup and replication jobs:

  • ESX(i) version 4.0 or later.

  • VM hardware version 7 or later.

  • Virtual machine storage must not be an independent disk (persistent or non-persistent).

  • I/O should go through virtualization layer (read ESX(i)).

NFS and RDM (in virtual compatibility mode, not in physical compatibility mode) are supported. VMFS is supported, whether backed by SAN, iSCSI, or local disk.

RCT makes it possible to copy only the data blocks that have changed since the last backup when an incremental backup of Hyper-V VMs is performed. RCT technology is supported on Hyper-V 2016 / 2019 Server and virtual machine configuration version 8 only.

Nutanix CRT is supported on Nutanix AHV cluster environments version or later.

It should be noted that enabling CBT/RCT/CRT affects VM performance (slightly, but to a measurable degree) as it consumes additional space on storage. Files with .rct and .mrt extensions are created when RCT is enabled for Hyper-V VMs. Files with .ctk extensions are created for each disk under CBT. Enabling of CBT/RCT/CRT also affects the performance of certain VM operations (like snapshot/checkpoint creation). Thus, it should be a well thought-out decision.

How to Enable or Disable CBT/RCT/CRT

You can enable/disable incremental block tracking on the Options page of the New Backup/Replication Job wizard:

To enable or disable change tracking for existing jobs, go to the product Jobs dashboard and configure the change tracking option through the Edit Job wizard.

  • No labels