What is Changed Block Tracking?

by SEP Blog Team | Virtual Backup, Changed Block Tracking

Most modern backup solutions for virtual environments utilize a technology called Changed Block Tracking (CBT). CBT is an incremental backup technology for virtual machines that creates faster and smaller backups.

The Benefits of CBT

  • Reduce the amount of time incremental backups take to perform
  • Free up disk space by only storing data new or changed data
  • Utilize incremental VM backups

CBT significantly reduces the amount of time a backup takes to perform because it tracks changes in a CBT log file. Using CBT, the VM API will determine if there was any change in the blocks since the last backup and will read only the blocks that have changed since the last snapshot or backup.

Therefore, third-party data protection products that communicate directly with the VM's API don't export unchanged data that has been previously backed up. CBT significantly reduces the amount of time and resources needed for the overall backup process by reducing backup windows and storage space. On average, the amount of data copied out is reduced by about 99%.


There are several VMs that offer CBT, most notably are VMware, Citrix XenServer and Microsoft Hyper-V. VMware CBT is derived from the vStorage Data Protection API and is part of the VMkernel storage stack. Changed blocks are recorded in a special log file at the VMDK level.

Microsoft developed Resilient Change Tracking (RCT) in order to provide native CBT for Hyper-V. RCT provides built-in CBT capability for Hyper-V VM disks and, just like VMware CBT, RCT tracks block level changes to a VM disk that can then be incrementally backed up by a third-party backup software.

Citrix XenServer 7.3 introduced CBT that is fairly similar to VMware. Citrix incremental backups were possible before this release, but they were cumulative and did not provide the rich functionality CBT brings to the table. Backup software can use the XenServer API to map changed blocks and read only blocks that have changed between snapshots.

Faster Backups with SEP

SEP offers the fastest snapshot and single file restore tools on the market. SEP backup and recovery software’s native communication with any virtual machine’s API enables easy-to-use CBT for VMware, Citrix XenServer and Microsoft Hyper-V (RCT) environments. SEP provides encrypted deduplication to further reduce the amount of data transferred and ensure data is secured at rest and in transit. CBT is just one of the many valuable tools SEP utilizes to create the most robust virtual backup solution.