queryChangedDiskAreas problem

Hi,

I am trying to query the changed blocks of a disk as described in the "designing Backup solutions for for VMware vSphere".

For whatever deviceKey I try I get an "invalid argument" error indicating that the deviceKey is wrong

Steps I do:

+create snapshot

+ wait till snapshot is done

+ try the queryChangedDiskAreas()

How can I get the deviceKey?

Thanks,

Nico