Enabling Block Level Incremental Backup

Note

Changed Block Tracking (CBT) is not supported when the virtual hardware version is 6 or earlier, or when the virtual disk is attached to a shared virtual SCSI bus.

For CBT to identify altered disk sectors since the last change ID, the following items are required:

The host must be ESX/ESXi 4.0 or later.
The virtual machine owning the disks to be tracked must be hardware version 7 or later.
I/O operations must go through the ESX/ESXi storage stack. So NFS is supported, as is RDM in virtual compatibility mode, but not RDM in physical compatibility mode. VMFS is supported, whether backed by SAN, iSCSI, or local disk.
CBT must be enabled for the virtual machine (see below).
Virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.

For CBT to identify disk sectors using Full backup, the following items are required:

The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk.
The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.

When using “Thick Provisioned Eager Zeroed” disks, the VMware CBT will report all blocks as “used” during the Full backup.

For virtual machines that do not support CBT, the vSphere Plugin will always perform a Full backup of the virtual disks.

To check if a virtual disk has CBT enabled, open the vSphere Client, select a powered-off virtual machine without snapshots.

Right-click the virtual machine and click Edit Settings.
Click the Options tab.
Click General under the Advanced section and then click Configuration Parameters. The Configuration Parameters dialog opens.
Click Add Row.
Add the ctkEnabled parameter and then set its value to true.
Click Add Row, add scsi0:0.ctkEnabled,and set its value to true.

Note

scsi0:0 in scsi0:0.ctkEnabled indicates the SCSI device assigned to the hard disk that is added to the virtual machine. Every hard disk added to the virtual machine is given a SCSI device that appears similar to scsi0:0, scsi0:1, or scsi1:1.

Note

VMWare FAQ articles may help:

During the first Full backup, the vSphere Plugin will try to enable CBT automatically. To disable this feature, use the keep_cbt option in the Plugin command string.

Plugin = "vsphere: keep_cbt host=guest1"

No snapshots must exist on a virtual machine at the time of enabling CBT on it.

If you notice large Full backup jobs despite small real disk usage, contact your Bacula Systems support team to assess the situation and possibly shrink the backup size by resetting CBT.

Detecting when CBT Is Not Available

When the CBT option is not available for a disk, the vsphere-ctl*log file may contain the following notice:

com.cybozu.vmbkp.soap.SnapshotManager, getChangedBlocksOfDisk]
      com.vmware.vim25.FileFault

When the vSphere Plugin receives this notice, it will automatically do a Full backup of the disk image.