Convert the virtual disk to Eager Zeroed Thick:

• If the VMDK virtual disk is of type Thin, convert the disk to Eager Zeroed Thick using the vmkfstools --inflatedisk command.      
       
       For example:
       
       vmkfstools --inflatedisk /vmfs/volumes/DatastoreName/VMName/VMName.vmdk
       
       Note: The inflate command reports the percentage of completion.

• If the VMDK virtual disk is of type Thick, convert the disk to Eager Zeroed Thick using the vmkfstools --eagerzero command.      
       
       For example:

  
        vmkfstools --eagerzero /vmfs/volumes/DatastoreName/VMName/VMName.vmdk
       
       Note: The eager zero command reports the percentage of completion.

Enabling clustering features for an existing virtual disk by converting in place (1035823)

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1035823

Convert the virtual disk to Eager Zeroed Thick