CBT Reset Observed on VM Due to Backup Application API Calls

Products

• VMware vSphere ESXi

Issue/Introduction

A VMware virtual machine experience unexpected changes to the Changed Block Tracking (CBT) configuration, impacting backup operations and RPO. Specifically, CBT was toggled from enabled → disabled without direct administrative action.

• Events show the VM being reconfigured -  config.changeTrackingEnabled: true -> false (Example event extracted from journalctl:):
  
  Month DD HH:MM:SS <vCenter_FQDN> vpxd[33157]: Event [205598701] [1-1] [YYYY-MM-DDTHH:MM:SS.] [vim.event.VmReconfiguredEvent] [info] [<domain/user_name>] [Syng-DC] [205598700] [Reconfigured <VM_Name> on <ESXi_Host_FQDN> in Syng-DC.
                                                      Modified:
                                                      config.changeTrackingEnabled: true -> false;
                                                       Added:
                                                       Deleted:
                                                      config.extraConfig("ctkEnabled"): (key = "ctkEnabled", value = "TRUE");
                                                      config.extraConfig("scsi0:0.ctkEnabled"): (key = "scsi0:0.ctkEnabled", value = "TRUE");
                                                      ]
.
.
.   
Month DD HH:MM:SS <vCenter_FQDN> vpxd[33157]: Event [205628830] [1-1] [YYYY-MM-DDTHH:MM:SS.] [vim.event.VmReconfiguredEvent] [info] [<domain/user_name>] [Syng-DC] [205628822] [Reconfigured <VM_Name> on <ESXi_Host_FQDN> in Syng-DC.
                                                      Modified:
                                                      config.changeTrackingEnabled: false -> true;
                                                       Added:
                                                      config.extraConfig("ctkEnabled"): (key = "ctkEnabled", value = "TRUE");
                                                      config.extraConfig("scsi0:0.ctkEnabled"): (key = "scsi0:0.ctkEnabled", value = "TRUE");
                                                       Deleted:
                                                      ]
  
  
  
• vpxd log event (/var/log/vmware/vpxd/vpxd.log)  shows Session login and reconfigure tasks match the users and source IPs associated with the backup tools.
  
        YYYY-MM-DDTHH:MM:SS. info vpxd[35614] [Originator@6876 sub=vpxLro opID=31bd3897] [VpxLRO] -- BEGIN task-8830483 -- vm-<vm_id>-- vim.VirtualMachine.reconfigure -- 5292b268-e3e6-ff9e-bda8-############(52c17ea6-e958-c247-18b1-############)
   YYYY-MM-DDTHH:MM:SS. info vpxd[35614] [Originator@6876 sub=vpxLro opID=31bd3897-01] [VpxLRO] -- BEGIN lro-2570878 -- vm-<vm_id>-- vim.VirtualMachine.reconfigure -- 5292b268-e3e6-ff9e-bda8-############(52c17ea6-e958-c247-18b1-############)
      YYYY-MM-DDTHH:MM:SS. info vpxd[35614] [Originator@6876 sub=VmCheck opID=31bd3897-01] CompatCheck results: (vim.vm.check.Result) [
        -->    (vim.vm.check.Result) {
        -->       vm = 'vim.VirtualMachine:b8aa5b8a-3771-4720-aa20-############:vm-<vm_id>',
        -->       host = 'vim.HostSystem:b8aa5b8a-3771-4720-aa20-############:host-id',
        -->    }
        --> ]
    YYYY-MM-DDTHH:MM:SS. info vpxd[35614] [Originator@6876 sub=VmProv opID=31bd3897-01] Applying ConfigSpec (vim.vm.ConfigSpec) {
        -->    createDate = "YYYY-MM-DDTHH:MM:SS.",
        -->    changeTrackingEnabled = false,


• Profiler output confirms the client IP making the API request (/var/log/vmware/vpxd/vpxd-profiler.log):
      vpxd-profiler-###.log:--> /SessionStats/SessionPool/Session/Id='5292b268-e3e6-ff9e-bda8-############'/Username='<user_name>'/ClientIP='<source_ip_address>'/PropertyCollector/FilterCount/total 0

Environment

vCenter Server 7.x
vCenter Server 8.x

Cause

CBT reset not triggered by VMware. Instead, it is initiated via authenticated API calls from external backup applications.

• Backup Software in some scenarios can reset CBT when backup metadata becomes invalid, snapshots are inconsistent, or a fallback to full/CRC mode is required.

• No internal vCenter faults or ESXi errors is observed.

• All configuration changes align with expected behavior of backup integration.

This confirms the CBT reset is externally triggered by backup software, not by VMware components.

Resolution

vCenter and ESXi are functioning as designed by applying API-driven configuration changes.

Customer Action Items

1. Engage Backup Support

   • Investigate reasons for CBT reset

   • Validate backup job configurations

   • Review backup logs for CBT inconsistencies or errors

2. Monitor VM Reconfigure Events

   • Track for any unexpected API operations

   • Validate that backup products are not conflicting

3. Re-baseline CBT if required

   • Perform a full backup after CBT reset

   • Check snapshot chain integrity

   • To disable/enable CBT on guest OS refer to the KB: Enabling or disabling Changed Block Tracking on Virtual Machine

Additional Information

 

• CBT is sensitive to changes in VM configuration, snapshots, or disk layout.

• Backup applications (not VMware) commonly issue CBT resets when inconsistencies are detected.

• This behavior is expected and intentional to preserve backup integrity.

For more details on CBT refer to the KB: changed-block-tracking-cbt-on-virtual-machine