Thanks but no thanks, this looks like what those "Hybrid" disks attempt to do, when in fact i would rather see those as two separate drives; that way i can place / and read only stuff in the SSD, and the bulk data in the mechanical device. Having some algorithm trying to "guess" what to place in the SSD is not efficient, this ain't no cache (and ram is better for that) :P

Also the i/o scheduler will get in the way, SSDs should use noop.