"Storage system design has always entailed a tradeoff between three parameters:" opines Kurt Marko in SearchStorage, "capacity, throughput and IOPS. Unfortunately, for systems engineers, the physical limitations of storage components don't allow them to be set independently -- at least, not in hardware.
Parametric dependency forces designers to choose between capacity and performance. Need more throughput? Throw more spindles or SSD controllers at the problem and live with the unused capacity..."