RAID groups based on endurance sets

The wear of storage devices in a storage array or storage server cluster is unleveled via modal Read/Write to create a plurality of endurance sets, each endurance set having devices with statistically similar amount of wear in a given period of time, and different endurance sets are subjected to different amounts of wear over a given period of time. The storage devices are organized into RAID groups such that multiple devices associated with the same RAID group are not members of the same endurance set. The number of devices in each endurance set may be selected to match the number of spare failover storage devices.