An embodiment is directed to a method for determining an assignment of data to be stored on at least one storage tier i of a plurality of storage tiers of a tiered storage system. The method including, for the at least one storage tier i, steps of accessing storage device characteristics of the at least one storage tier i of the plurality; based on the accessed storage device characteristics, splitting the at least one storage tier i into Ni storage subtiers of the at least one storage tier i, the Ni storage subtiers having respective storage device characteristics; and based on characteristics of data to be stored on the tiered storage system and the respective storage device characteristics of the Ni storage subtiers, determining an assignment of data to be stored on each of the Ni storage subtiers. Embodiments are directed to related methods, systems and computer program products.