Have you ever seen an error saying “changed block tracking must be enabled on the VM for this backup type”? If you manage virtual machines in VMware environments, this message is common—especially when setting up incremental or differential backups. It means your backup software needs a special feature called Changed Block Tracking (CBT) to work efficiently with these backup types. Let’s explore what CBT does, why it matters so much for modern backups, how to enable it step by step—and what to do if things go wrong.
What Is Changed Block Tracking?
Changed Block Tracking (CBT) is a VMware vSphere feature that tracks which parts of a virtual machine's disk have changed since your last backup operation. Instead of copying every byte of data each time you back up a VM, CBT lets your backup tool copy only those blocks that have actually changed since the last snapshot or backup job.
How does it work? When CBT is enabled on a VM and its disks, vSphere maintains a bitmap log file alongside each virtual disk—the CTK file—that records which disk blocks were modified after each snapshot or backup event. Backup applications can then query this log using VMware APIs to quickly identify just the changed data blocks instead of scanning entire disks.
CBT operates at the virtualization layer—it doesn’t require any changes inside your guest operating system—and works with VMs running hardware version 7 or later. This makes it efficient and reliable for most enterprise workloads.
Why Changed Block Tracking Must Be Enabled？
Why does “changed block tracking must be enabled on the VM for this backup type” appear so often? Because certain types of backups depend entirely on knowing exactly what has changed since your last run—otherwise they lose their efficiency advantage.
Incremental and differential backups are designed to save only new or modified data since either the previous full/incremental/differential/forever-incremental backup. Without CBT enabled at both VM and disk level, your backup application cannot tell which blocks have changed between jobs; it may fall back to scanning all data blocks manually—a slow process—or revert to taking full image-level backups every time.
This leads directly to longer backup windows, increased storage consumption, higher network load during transfers—and sometimes even failed jobs if strict incremental mode is required by policy or compliance rules. In short: enabling CBT isn’t just best practice; it’s essential for fast and reliable modern VM protection workflows.
Backup Types Requiring Changed Block Tracking
Not all VM backups need CBT—but many do if you want speed and efficiency:
Incremental Backups: These capture only changes made since the last successful incremental or full backup job.
Differential Backups: These capture all changes made since the last full backup.
Forever-Incremental Strategies: Some solutions use forever-incremental chains where only one initial full plus ongoing incrementals are stored; these rely completely on accurate change tracking.
Synthetic Full Backups: Some tools build new “full” restore points from chains of incrementals—again requiring precise knowledge of changed blocks over time.
Granular Restores: Fast restores often depend on being able to pinpoint exactly which files were affected between points in time using underlying block change logs.
Full image-level backups don’t require CBT—they simply copy everything—but enabling CBT can still help track used sectors more efficiently behind-the-scenes. If you try an incremental/differential job without CBT active at both levels (VM + disks), expect errors like “changed block tracking must be enabled on the vm for this backup type”—or silent fallback to slower methods that waste resources.
How to Enable Changed Block Tracking Using vSphere Web Client？
Enabling CBT through vSphere Web Client takes careful attention but follows clear steps:
First—make sure your target virtual machine is powered off with no snapshots present! Existing snapshots prevent proper activation of change tracking.
1. In vSphere Web Client, right-click your powered-off virtual machine and select Edit Settings.
2. Go to the VM Options tab.
3. Expand Advanced settings and click Edit Configuration.
4. Click Add Configuration Params.
5. Add a parameter named ctkEnabled with value set as TRUE.
6. For each virtual disk attached (for example scsi0:0), add another parameter named like scsi0:0.ctkEnabled set as TRUE (replace "scsi0:0" with actual SCSI device name per disk).
7. Click OK to save changes.
8. Power on your virtual machine again once finished editing parameters.
Tip: The per-disk parameter (scsiX:X.ctkEnabled) is critical—without it set correctly for every attached disk you wish protected via incremental/differential jobs, those disks won’t generate CTK files needed by most modern backup tools!
After powering up again check your datastore folder for files ending in *-ctk.vmdk next to each VMDK file; their presence confirms successful activation of changed block tracking per disk device.
If you see duplicate parameters already present (for example from earlier attempts), make sure their values are set correctly rather than adding redundant entries—which could cause confusion later during troubleshooting!
How to Enable Changed Block Tracking Using PowerCLI？
For larger environments—or anyone who prefers automation—PowerCLI offers an efficient way to enable CBT across many VMs at once:
Again ensure VMs are powered off with no snapshots before proceeding!
1. Open PowerCLI shell then connect using
Connect-VIServer <your_vcenter_server>
2. Retrieve target VM object
$vm = Get-VM -Name "YourVMName"
3a. Set global ChangeTracking flag
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.ChangeTrackingEnabled = $true
$vm.ExtensionData.ReconfigVM($spec)
3b (Advanced): For each hard disk attached configure per-disk flag:
foreach ($hd in $vm.ExtensionData.Config.Hardware.Device | Where { $_ -is [VMware.Vim.VirtualDisk] }) {
$key = "$($hd.ControllerKey):$($hd.UnitNumber).ctkEnabled"
New-AdvancedSetting -Entity $vm -Name $key -Value "TRUE" -Force
}4. Power up your VM again using
Start-VM -VM $vm
This approach ensures both global (ChangeTrackingEnabled) and individual (scsiX:X.ctkEnabled) flags are properly configured—even across multiple disks per server! Afterward always verify creation of *-ctk.vmdk files within datastore folders corresponding to protected VMs/disks before scheduling production jobs relying on incremental/differential modes.
Automation tip: You can loop through multiple VMs by wrapping above logic inside standard PowerShell loops targeting lists retrieved via Get-VM.
How to Back Up Virtual Machines With Vinchin Backup & Recovery？
Once Changed Block Tracking is active, leveraging advanced enterprise-grade protection becomes straightforward with Vinchin Backup & Recovery—a professional solution supporting over 15 mainstream virtualization platforms including VMware, Hyper-V, Proxmox, oVirt, OLVM, RHV, XCP-ng, XenServer, OpenStack, ZStack and more. For VMware users especially, Vinchin Backup & Recovery utilizes features such as quiesced snapshot integration and HotAdd transport mode alongside robust support for Changed Block Tracking itself—delivering high-speed incremental backups tailored precisely for demanding environments.
Among its extensive capabilities are forever-incremental backups utilizing deduplication and compression; granular restore options; instant recovery; LAN-free transfer paths; scheduled automation—all designed around maximizing efficiency while minimizing resource impact across diverse infrastructures regardless of scale or complexity.
Backing up a VMware VM using Vinchin Backup & Recovery involves just four intuitive steps within its web console:
Step 1. Select Backup Source
Step 2. Select Backup Destination
Step 3. Select Backup Strategies
Step 4. Review and submit the job
Recognized globally by thousands of organizations—with top industry ratings—Vinchin Backup & Recovery offers a fully-featured free trial valid for 60 days so you can experience true enterprise-class data protection firsthand.
Changed Block Tracking FAQs
Q1: Can enabling changed block tracking impact performance during heavy write activity?
A1: In rare cases high-I/O workloads may experience slight overhead from extra logging operations but most production systems show negligible impact according official documentation from Broadcom/Vmware sources.
Q2: What should I do if my automated script fails because some disks lack ctk-enabled parameters?
A2: Review script output/logs carefully then manually verify/add missing per-disk keys matching exact controller/unit numbers shown under Edit Settings > Hard Disks > Advanced Parameters section within vCenter UI before retrying batch runs again afterward!
Q3: Is there any risk disabling/re-enabling ctk flags repeatedly over time?
A3: No significant risk exists provided all actions performed while powered off/no active snapshots present—but always test non-production clones first when possible prior applying bulk updates across live infrastructure!
Conclusion
Changed Block Tracking must be enabled on the VM for this backup type if you want fast reliable incremental protection in modern environments—especially at scale! Enabling it ensures smooth operation whether automating via scripts/UI tools alike while minimizing resource usage long-term too! Vinchin supports seamless integration leveraging these capabilities out-of-box so give us try today risk-free!
