how to get valid blocks in vmdk extent file

In Esxi 6.0, I create a vm in NFS store, every virtual disk of the vm has disk_name.vmdk  and  disk_name-flat.vmdk.

disk_name.vmdk:

# Disk DescriptorFile

version=3

encoding="UTF-8"

CID=164e2cb0

parentCID=ffffffff

isNativeSnapshot="no"

createType="vmfs"

# Extent description

RW 8388608 VMFS "disk_name-flat.vmdk"

# The Disk Data Base

#DDB

ddb.adapterType = "lsilogic"

ddb.deletable = "true"

ddb.geometry.cylinders = "522"

ddb.geometry.heads = "255"

ddb.geometry.sectors = "63"

ddb.longContentID = "2cd42eda8ae3ff5540338b8f164e2cb0"

ddb.thinProvisioned = "1"

ddb.uuid = "d7 07 97 90 a3 6a 48 30-a0 fc d9 55 b9 6d ba 1f"

ddb.virtualHWVersion = "11"

I find https://www.vmware.com/support/developer/vddk/vmdk_50_technote.pdf , vmdk file format is discribled.

But I find my disk_name-flat.vmdk is inconsistent with pdf file.

The beginning of the disk_name-flat.vmdk is zero bit.

But in vmdk_50_technote.pdf , the beginning of the file may be ESXi Host Sparse Extent Header, why?

My real purpose is to get valid data on disk by parsing the contents of the vmdk file ,  I hope someone can help me, thanks very much!