A method is used in managing data on data storage systems. Rules are defined for candidate data unit selection. The rules are automatically executed on a primary data storage system to examine data units on the primary data storage system. The rules are applied to the data units. A data unit is identified that matches the rules and the identified data unit is migrated. The identified data unit is copied from the primary storage system to a secondary storage system. The identified data unit on the primary storage system is replaced with metadata. the data storage space in the primary storage system corresponding to the identified data unit is freed up.