Summary
Although agentless VMare backup is designed to back up only allocated blocks, in some cases the entire VM disk is backed up. These cases are described in a VMware knowledge base article.
Symptoms
The job log displays a warning message similar to the following -
Failed to get allocated bitmap for disk [datastore1] Win2008-x64-Patch/Win2008-x64-Patch-000003.vmdk reason Unknown reason, proceeding with full bitmap.
Resolution
The above message is redirected from VMware and is documented in the VMware knowledge base at kb.vmware.com/kb/1020128
The following, excerpted from the VMware knowledge base article, may be relevant to this case:
For CBT to identify disk sectors in use with the special "*" change ID, the following items are required:
The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk. RDM is not VMFS.The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.