1. A method performed by a set of processors for replacing storage elements in a storage array, the method comprising: selecting, in accordance with a selection process, one of multiple storage element types for replacing a storage element in a redundant group of storage elements of the storage array, wherein the selection process has a biasing that is adjustable to change the probability of selecting any of the storage element types relative to the others; adding a storage element of the selected storage element type to the redundant group; operating the redundant group with the added storage element of the selected storage element type; calculating a service level of the redundant group in response to operating the redundant group with the added storage element of the selected storage element type; and adjusting the biasing of the selection process, in response to calculating the service level of the redundant group after selecting the selected storage element type, to vary the probability that the selected storage element type will later be selected according to the selection process when subsequently selecting one of the storage element types for replacing a storage element of the redundant group.