1. A method comprising: in a computer cluster in communication with a storage system, the storage system organized in a hierarchy, assigning a different role to each storage tier within the hierarchy, each role related to a workload characteristic; arranging the tiers according to a number of cycles required to access a tier; managing the tiers across the cluster, the management responsive to a flexible tier definition within the hierarchy and the tier arrangement; and selecting a first storage tier for placement of a data object, and placing the data object in the first tier, the selection responsive to a calculated probability of access of the object and the number of cycles required to access the tier.