VMDK internal format

Does anyone know if it is possible to read the internal format of a VMDK?


For thin provisioned VMDks I would like to do O/S disk free space reclamation by mapping the O/S free space clusters and the VMDK grain tables  to see which blocks would need to be written as binary zeros so that the VMDK could be shrunk to it's minimum.


The windows sdelete utility would work but that writes to every unused cluster thus inflating the VMDK to it's maximum size.


Thanks.


Bob