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

Article:TECH197311  |  Created: 2012-09-26  |  Updated: 2013-07-26  |  Article URL
Article Type
Technical Solution


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.


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>


Follow these steps to reset CBT on a guest virtual machine.

Note: Before doing these steps verify that the virtual machine does not have any current snapshots and the virtual machine is Hardware Version 7 or higher.

1. Power off the virtual machine

2. Righ click on the virtual machine and select "Edit Settings". Select the "Options" tab and click the box for "Configuration Parameters"

3. Locate "ctkEnabled" and change the value to false

4. Locate "scsi0:x.ctkEnabled" and change the value to false.  (where "x" is the number for the vmdk file, example scsi0:0.ctkEnabled would be the first VMDK for the virtual machine)
Note: There may be multiple listings for this entry if the virtual machine has multiple VMDK files.  All entries should be set to false.

5. Open the folder the virtual machine is stored in on the datastore and remove any -CTK.VMDK files

6. Power on the virtual machine

7. After the virtual machine has completed it's boot sequence, power the virtual machine off. Note: This step is required to update the CTK tables for the virtual machine.

8. Power on the virtual machine.

9. Run the backup of the virtual machine.  When this backup runs CBT will get enabled. (If the backup is scheduled to run at a later time CBT will get enabled at that time)

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