Maximizing track utilization by reblocking data sets

DFSMSdss provides a REBLOCK keyword that allows users to maximize the track usage by data sets during copy and restore processing. When REBLOCK is specified on a fully or partially qualified name of a sequential or partitioned data set during copy or restore processing, DFSMSdss will choose an optimal block size for the data set and the device. However, the installation reblock exit can specify that a different block size be used (except for partitioned load modules during copy operations).

REBLOCK is ignored for:

Unmovable data sets (unmovable attribute in the DSORG field in the VTOC entry)
Data sets with record format of U (except for partitioned load modules during copy operations), V, VB, VBS, or F
Partitioned data sets with note lists (except for partitioned load modules during copy operations)
Partitioned data sets that are also specified in the NOPACKING keyword

Partitioned load modules can be reblocked in copy operations, even if they have note lists.

The reblockable indicator in the data set’s VTOC entry also determines whether a data set is to be reblocked. When the indicator is on, the data set is always reblocked to a system determined, optimal block size, except when the data set:

Is a partitioned data set that is also specified in the NOPACKING keyword
Is an unmovable data set
Has a record format of V, VS, VBS, or F

The installation reblock exit is not called if the reblockable indicator is ON.

A PDSE being converted to a PDS will always be reblocked except when the data set has a record format of V, VS, VBS, or F; or when the data set’s record length is ‘0’.

For more information about the installation reblock exit routine, refer to z/OS DFSMSdfp Storage Administration.