Data migration methodology for use with arrays of powered-down storage devices

A method for managing extents in a data storage system includes monitoring usage statistics for an extent residing on one or more powered-up storage devices. In the event the extent has not been accessed for specified period of time (as determined from the usage statistics), the method automatically compresses the extent and migrates the extent to an intermediate repository. Once the amount of data in the intermediate repository reaches a specified level, the method migrates the extent from the intermediate repository to one or more normally powered-down storage devices. If I/O is received for the extent while it resides in the normally powered-down storage devices or the intermediate repository, the method automatically migrates the extent from the normally powered-down storage devices or the intermediate repository to the normally powered-up storage devices. A corresponding apparatus and computer program product are also disclosed.