Documentation

Incremental backup doesn't work

Causes and solutions as a regular checklist for VMware Changed Block Tracking (CBT) problems

1. Basic requirements

  1. The virtual machines must not have any VMware snapshots.
  2. When using vSphere ESXi 5.1 to vSphere ESXi 5.5 Update 2, the CBT is reset with active Storage vMotion. You can find more information about this problem here: https://kb.vmware.com/s/article/2048201
  3. In some cases after a virtual machine crash, the CBT can be reset.
  4. If ESXi 6.0.0 is used, at least build 15169789 or newer must be installed. There are CBT problems with older ESXi builds.


If you have checked all of the points and the problem persists, see the next section for instructions on how to manually activate CBT.


2. Enable VMware Changed Block Tracking (CBT)

  1. The virtual machine must be shut down before making these changes. There must be no VMware snapshots with this virtual machine, delete and consolidate previously existing VMware snapshots.
  2. Log in to the vSphere Client or vSphere WebClient.
  3. Select the virtual machine. Right-click and choose "Edit settings".
  4. Click "VM Options", then click "Advanced".
  5. In the "Configuration parameters" section, click "Edit configuration".
  6. Click on "Add parameters" and enter the following values:
    1. Key: ctkEnabled
    2. Value: true
  7. Depending on the number of volumes connected via SCSI add further parameters.
    1. Example: Virtual hard disk on the first SCSI controller, enter scsi0:0.ctkEnabled as "key" and true as "value".
    2. In the case of three further virtual hard disks, the next parameters would be "key" scsi0:1.ctkEnabled, scsi1:0.ctkEnabled, scsi1:1.ctkEnabled etc. all with the "value" true.
  8. Start the virtual machine.
  9. Check the datastore and the directory of this virtual machine for new files with the *-ctk.vmdk extension.
  10. Start the incremental backup.


3. Disable and reset VMware Changed Block Tracking (CBT)

  1. The virtual machine must be shut down before making these changes. There must be no VMware snapshots with this virtual machine, delete and consolidate previously existing VMware snapshots.
  2. Log in to the vSphere Client or vSphere WebClient.
  3. Select the virtual machine. Right-click and choose "Edit settings".
  4. Klicken Sie auf "VM-Optionen" und anschließend auf "Erweitert".
  5. In the "Configuration parameters" section, click "Edit configuration".
  6. Enter ctkEnabled in the search field for "Key". All CBT parameters should have their values set to true.
  7. Now change the "value" of all CBT parameters to false and make sure that all virtual disks of the virtual machine are changed.
  8. In the datastore and directory of this virtual machine, delete all files with the extension *-ctk.vmdk.
  9. Start the virtual machine.


After these changes, incremental backups are no longer possible. To reactivate CBT afterwards, proceed as described in point 2.

Additional notes from VMware for this topic can be found here: https://kb.vmware.com/s/article/1020128, https://kb.vmware.com/s/article/1031873 and https://kb.vmware.com/s/article/2139574