A data preservation function is provided which, in one embodiment, includes indicating by a map, usage of a particular map extent range by a relationship between a source extent range of storage locations on a source storage device containing data to be preserved in the source extent range, and a target extent range mapped to the map particular extent range. In another aspect, in response to receipt of a data preservation command, a data preservation operation is performed including determining whether a map indicates availability of a map extent range mapped to the identified target extent range. Upon determining that a particular map indicates availability of a map extent range mapped to the identified target extent range, a relationship between the identified source extent range and the identified target extent range is established. Other features and aspects may be realized, depending upon the particular application.