MySQL Scale-Out by application partitioning by Oli Sennhauser
Eventually every database system hit its limits. Especially on the Internet, where you have millions of users which theoretically access your database simultaneously, eventually your IO system will be a bottleneck. [A] promising but more complex solution with nearly no scale-out limits is application partitioning. If and when you get into the top-1000 rank on alexa [1], you have to think about such solutions.
Horizontal application partitioning, Vertical application partitioning, Disk IO calculations, How to partition an entity