Releasing Space

The close function attempts to release unused tracks or cylinders for a data set if all of the following are true:

The SMS management class specifies YI or CI for the partial release attribute, or you specified RLSE for the SPACE parameter in the DD statement or RELEASE in the TSO ALLOCATE command.
You did not specify TYPE=T on the CLOSE macro.
The DCB was opened with the OUTPUT, OUTIN, OUTINX, INOUT or EXTEND option and the last operation before CLOSE was WRITE (and CHECK), STOW or PUT.
No other DCB for this data set in the address space was open.
No other address space in any system is allocated to the data set.
The data set is sequential or partitioned.
Certain functions of dynamic allocation are not currently executing in the address space.

For a multivolume data set that is not in extended format, or is in extended format with a stripe count of 1, CLOSE releases space only on the current volume.

Space also can be released when DFSMShsm is performing space management or when an authorized program issues the PARTREL macro.

Space is released on a track boundary if the extent containing the last record was allocated in units of tracks or in units of average record or block lengths with ROUND not specified. Space is released on a cylinder boundary if the extent containing the last record was allocated in units of cylinders or in units of average block lengths with ROUND specified. However, a cylinder boundary extent could be released on a track boundary if:

The DD statement used to access the data set contains a space parameter specifying units of tracks or units of average block lengths with ROUND not specified, or
No space parameter is supplied in the DD statement and no secondary space value has been saved in the data set label for the data set.

Changing a cylinder boundary extent to a track boundary extent generally causes loss of the possible performance benefit of a cylinder boundary. On the latest disk drives there is no performance benefit of cylinder boundaries.