Real-time data migration is key
I haven't come across an auto-tiering solution that boasts real-time data movement. Most are based on a batch process which makes no sense at all as yesterdays 'hot' data is being moved to the most expensive storage tier. As Chris points out, small block data movement provides the highest efficiency but the movement needs to occur in as close to real-time as possible with minimal overhead to I/O performance. [Disclosure: I work for Dot Hill Systems]