There will be a moment in your life when your DataStore will run out of space or your ESX 3.5 Virtual Machine should be exported to a new brand ESX 4.0 server and you shall decide to take advantage of Thin Provisioning. That day you’ll need to deal with ‘vmkfstools’.
Basically the process is rather simple but your VM must be powered off and must have the same free space in a datastore as your current vmdk file size. Then just connect to your ESX console – putty’s time – become su and run this command:
vmkfstools -i /vmfs/volumes/VMname/VMname.vmdk /vmfs/volumes/VMname/VMname-thin.vmdk -d ‘thin’ -a lsilogic
The cloning process will start and you’ll get a thin copy of your thick vmdk file. Let’s edit your VM from VI client to remove -without deleting- the old disk and add your thin one as an existing disk. Check that your VM starts and runs properly. Then shut it down and browse your VM datastore, check how much storage is really using your new thin vmdk file. Once recovered from the shock 🙂 delete your old vmdk file.