Embodiments of the invention relate to cluster-centric tiered storage with a flexible tier definition to support performance of transactions. Object data is distributed in a multi-tiered shared-nothing cluster. Hierarchical tiers of data storage are assigned different roles within the hierarchy. The tiers are managed globally across the cluster and objects are placed in tiers according to a flexible tier definition. The probability of object access is computed for objects, and objects are moved to different tiers responsive to the computation to minimize system runtime. The location of an object is further optimized in response to an access request.