A backed up or migrated data set rarely ends at the exact physical end of a tape. Data sets often span tapes because as a data set is copied, the tape is filled, another tape is requested, and any remaining data is copied to the next tape. Because the data set now spans these two tapes, these tapes are logically related.

A group of DFSMShsm tapes that are logically related is called a connected set. For backup and migration tapes, this relationship exists because data sets can span tapes. For dump tapes, this relationship exists because the tapes are part of a dump copy. Because connected sets can be large in size, DFSMShsm provides a method of reducing the size of a connected set by reducing occurrences of data sets that span physical tape volumes. The creation of data sets that span tapes in a connected set (spanning data sets) can be reduced with the SETSYS TAPESPANSIZE command, which is discussed in detail in z/OS DFSMShsm Storage Administration.