Change Block Tracking: What It Is and How It Works?

As virtual infrastructures grow larger, traditional full backups quickly become inefficient. Backing up entire virtual disks repeatedly consumes unnecessary storage, increases backup Windows, and generates heavy I/O workloads.

To solve this problem, modern virtualization platforms introduced Change Block Tracking (CBT).

Change Block Tracking identifies exactly which disk blocks have changed since the last backup, allowing backup software to process only modified data. This dramatically improves backup performance, reduces storage consumption, and shortens backup windows.

What Is Change Block Tracking?

Change Block Tracking (CBT) is a virtualization feature that records which disk blocks of a virtual machine have changed since the previous backup or snapshot.

Instead of scanning the entire disk, CBT maintains metadata describing modified blocks. Backup software can query this information and back up only the data that has changed.

Why Change Block Tracking Is Important?

Without CBT, incremental backup typically works like this:

• Scan the entire virtual disk
• Identify modified data
• Transfer changed files or blocks

For large VMs, this process wastes time and computing resources.

With CBT enabled:

• The hypervisor records disk changes automatically
• Backup software retrieves the changed block list
• Only modified blocks are transferred

This dramatically improves backup efficiency.

How Change Block Tracking Works

Change Block Tracking (CBT) works by monitoring changes at the block level on a virtual disk. Instead of scanning the entire disk during each backup, CBT keeps track of which blocks have been modified since the last backup, allowing incremental backups to process only the changed data.

Here’s how it works step by step:

1. Monitoring Disk Writes – CBT hooks into the virtualization layer and records all write operations to the virtual disk. Each block that is modified is flagged in a change-tracking metadata file.
2. Storing Change Metadata – The metadata, usually stored alongside the virtual disk, maintains a record of all changed blocks. This allows backup software to quickly identify which blocks need to be backed up.
3. Incremental Backup Query – When an incremental backup is triggered, the backup software queries the CBT metadata to retrieve the list of changed blocks.
4. Backup Only Changed Blocks – Using this information, only the modified blocks are transferred and stored, greatly reducing backup time, network usage, and storage overhead.

CBT is widely used in virtualized environments such as VMware and Hyper-V, where it significantly improves the efficiency of incremental backups while reducing the impact on VM performance.

Change Block Tracking VMware Architecture

In VMware environments, CBT operates inside the hypervisor storage layer.

The typical workflow looks like this:

• Backup software sends request
• Hypervisor checks change tracking metadata
• Changed block list is returned
• Backup system transfers only those blocks

Because CBT runs at the hypervisor level, it does not require agents inside virtual machines.

Key Benefits of Change Block Tracking

Change Block Tracking (CBT) brings significant advantages for organizations managing virtualized environments. By tracking only the blocks that have changed since the last backup, CBT drastically reduces the amount of data that needs to be processed, which in turn speeds up incremental backups and shortens backup windows. This not only saves time but also lowers the load on storage and network resources, making backups more efficient and less disruptive to ongoing operations.

Moreover, CBT improves scalability for large environments. Instead of performing time-consuming full-disk scans, IT teams can rely on CBT to handle even extensive virtual infrastructures with minimal impact on performance. When paired with backup solutions like i2Backup, CBT enables organizations to implement high-performance, resource-efficient backup strategies, ensuring data protection without compromising system availability.

CBT vs Snapshot vs Incremental Backup

Feature	Change Block Tracking	Snapshot	Incremental Backup
Purpose	Track changed disk blocks	Capture VM state	Store changed data
Layer	Hypervisor metadata	Storage layer	Backup software
Data Stored	Change map only	Full VM state	Changed blocks
Performance Impact	Low	Moderate if long-term	Low

These technologies are complementary rather than competing. CBT identifies changes, snapshots capture system state, and incremental backups store the modified data.

VMware CBT vs Hyper-V RCT

Feature	VMware CBT	Hyper-V RCT
Platform	VMware	Microsoft Hyper-V
Tracking Method	CTK metadata files	Hyper-V change log
Introduced In	vSphere 4	Windows Server 2016
Purpose	Optimize incremental backup	Optimize incremental backup

How to Enable Block Change Tracking

Enabling Changed Block Tracking (CBT) in VMware environments allows backup software to efficiently identify which blocks of a virtual disk have changed since the last backup, significantly speeding up incremental backups. Follow these steps to enable CBT safely and correctly:

Step 1. Verify VM Requirements
Before enabling CBT, ensure the virtual machine meets the necessary conditions: it should be powered on, have virtual hardware version 7 or higher, and contain no active snapshots. If the VM has snapshots or unsupported disk configurations, CBT may fail to initialize, which could trigger full backups instead of incrementals.

Step 2. Access VM Settings
Open the VMware vSphere Client, right-click the target VM, and select Edit Settings. Go to the VM Options tab, expand Advanced, and open Configuration Parameters.

Step 3. Enable CBT for the VM and Disks
Check if the following parameters exist:

• ctkEnabled = "TRUE" – enables CBT for the VM.
• scsiX:Y.ctkEnabled = "TRUE" – enables CBT for each virtual disk, replacing X:Y with the SCSI controller and unit numbers.

If these parameters are missing, add them manually. Saving the configuration will create -ctk.vmdk files in the datastore, which store the change block metadata.

Step 4. Perform a Baseline Full Backup
Once CBT is enabled, run a full backup immediately. This establishes a baseline for all future incremental backups. Skipping this step may result in incomplete or inconsistent incrementals.

Step 5. Avoid Actions That Reset CBT
Be cautious with operations such as virtual hardware changes, deleting snapshots, or VM migrations. These actions can reset or invalidate CBT metadata, requiring re-enabling and a new full backup.

Using CBT with i2Backup

Many enterprise backup solutions can leverage CBT to accelerate incremental backups. For example, Info2Soft’s i2Backup integrates seamlessly with CBT, enabling faster, resource-efficient backups in virtualized environments.

Enterprise backup platforms such as i2Backup from Info2Soft leverage change block tracking to significantly accelerate incremental backup operations. Instead of scanning entire virtual disks, i2Backup queries the hypervisor for changed block information and transfers only the modified data.

This approach allows organizations to protect large VMware environments more efficiently while reducing storage consumption and backup Windows.

When Change Block Tracking May Reset

In some cases, CBT metadata may reset, including:

• Host crashes
• Disk configuration changes
• VM hardware upgrades
• Snapshot inconsistencies

If CBT resets, the next backup usually runs as a full backup.

Frequently Asked Questions

Conclusion

In summary, Changed Block Tracking (CBT) is a powerful technology that allows backup software to identify which blocks of a virtual disk have changed since the last backup. By enabling CBT, organizations can significantly reduce backup time, minimize I/O usage, and improve overall backup efficiency. It is important to ensure that the virtual machine meets the prerequisites and to perform a full backup after enabling CBT to establish a baseline. Using backup solutions like i2Backup can further maximize the benefits of CBT in enterprise environments.