1. An apparatus comprising: a data storage system comprising: a plurality of non-transitory data storage devices each having a performance capability, where ones of the storage devices having a common performance capability are grouped together in a storage pool at a storage tier and the data storage system includes multiple storage tiers; at least one storage director which presents logical storage devices to an application, the logical storage devices being backed by the non-transitory storage devices; and a data movement engine configured, for each storage pool, to: assign one state of a plurality of states to the storage pool based on demanded activity and demanded capacity, the plurality of states including a healthy state in which demanded activity and demanded capacity are acceptable for the storage pool, a first degraded state in which demanded activity is unacceptable for the storage pool, and a second degraded state in which demanded capacity is unacceptable for the storage pool; where the storage pool is in the first degraded state, promote extents from that storage pool to a different storage pool of a tier with greater performance capability in order to cause that storage pool in the first degraded state to be in the healthy state; and where the storage pool is in the second degraded state, demote extents from that storage pool to a storage pool of a tier with lesser performance capability in order to cause that storage pool in the second degraded state to be in the healthy state.