1. Open MOB(Managed Object Browser) web site at https://your_vcenter_or_esxi/mob
2. Navigate to virtual machine object
3. Additionally create a snapshot for the virtual machine
4. Click on "QueryChangedDiskAreas"

5. Insert the following parameters:
snapshot reference id
device key of a virtual disk
change id (use "*"(star symbol) to query all used space)
start offset (the beginning of the required changed area)

7. Click "Invoke Method" and then result will be shown at the bottom in the same window.
The table contains changed disk areas start offsets and lengths.
