Reading only used blocks

A call to VixDiskLib_Clone allows to create a thin provisioned disk. If I loop over the disk via VixDiskLib_Read I also back get unused sectors. If I use the vStorage call QueryChangedDiskAreas with "*" this works but only for disk on vmfs storage.

How could I "emulate" a VixDiskLib_Clone with calls to VixDiskLib_Read and copy a thin provisioned disk?