It seems to me that when dirmaint is left at defaults for flashcopy behavior 2 that it appears to adopt a polling process to check whether the storage device has actually finished copying the volume in the background before returning from DVHDXD. It essentially turns the instantaneous point in time copy to a synchronous copy, like a fast DDR.
This is one of the things that makes STK's copy-on-write approach to snapshots so nice. You can capture the snapshot at a point in time, and then the controller can fill in the blanks in the background. Much nicer for performance and management. It also doesn't take up space until some machine actually uses the disk block.
