Requesting Partial Release of DASD Data Set Space

The best way to control partial release is to assign SMS management classes to data sets. The PARTIAL RELEASE management class attribute is easier to define and maintain than using IFG0EX0B, and the management classes give you greater flexibility in defining and changing your partial release policy. You can modify the JFCB in the open work area to specify partial release, as shown in Example of the IFG0EX0B Exit Routine. The example sets JFCRLSE (bits 0 and 1; mask X'C0') to 1 in the JFCBIND1 field, indicating a request for partial release. Do this only for DASD physical-sequential data sets, sequential extended-format data sets, or partitioned data sets opened for OUTPUT, OUTIN, EXTEND, and OUTINX and processed by one of the following:

EXCP with a 5-word device-dependent section present in the DCB
BSAM
QSAM
BPAM

Be careful when you change the JFCB release bits. If you frequently open a data set for output and write varying amounts of data each time, DADSM can extend the data set after each OPEN and create many small extents, and perhaps reach the extent limit for the data set. This could result in a B37 abend.

Also, use caution when setting the JFCBSPAC bits to define the space quantity units when the partial release flag, JFCBRLSE, is also set on.A cylinder-allocated extent can be released on a track boundary if JFCBSPAC does not indicate cylinder units or average block length units with ROUND specified. This causes the cylinder boundary extent to become a track boundary extent, thereby losing the performance advantage of cylinder boundary extents. Zeroing the release indicator and increasing secondary allocation quantity (for example, when the data set has extended a large number of times) can prevent such a B37 abend. Setting the release indicator could result in more space being made available to other users sharing the volume.