How to reset Change Block Tracking (CBT) for VMware Backups


During backups of virtual machines the VMware API's may have problems collecting the change block tracking information of a specific virtual machine.  When this occurs the entire VMDK will be backed up, not just the used space.  When this occurs it may be neccesary to reset the Change Block Tracking (CBT) for the virtual machine.

At the time of writing this article VMware themselves had acknowledged that a CBT reset may be required if issues are seen during backup operations in VMware KB 2048201 ( )

Error Message

From the beremote debug log:

[fsys\vmvcb]         - SymVmTools: GetDiskChangedInfoAPI: SYM_VMC_ERROR:  SOAP_ERROR
[fsys\vmvcb]         - SymVmTools: SOAP 1.1 fault: "":ServerFaultCode [no subcode]
"Error caused by file /vmfs/volumes/DATASTORE/VIRTUAL MACHINE/VIRTUAL DISK.vmdk"
Detail: <FileFaultFault xmlns="urn:vim25" xsi:type="FileFault"><file>/vmfs/volumes/DATASTORE/VIRTUAL MACHINE/VIRTUAL DISK.vmdk</file></FileFaultFault>


To reset the CBT:
1)  Disable the CBT by following the steps documented by VMware in KB 1031873 (make sure you are following the disable CBT steps and also be aware, that the additional information section of the KB, documents a method that requires PowerCLI but does not require a restart of the VM and therefore provides a choice of methods for  those familiar with PowerCLI)
2) Use Backup Exec to run a full backup after disabling CBT 



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