Data sets with undefined (U) record formats do not follow the same rules as those with other record formats. In particular, most load libraries in partitioned data sets (not PDSEs) will require less space (often as much as 20% less) and offer better performance at increasing block sizes up to the block size limit of 32,760 bytes. This is because the program management binder, linkage editor, and IEBCOPY COPYMOD command use the data set block size only to set the maximum block length they will use. They will write a block whenever the space available on a track is greater than the minimum block size (over which you have no control) and less than or equal to the maximum block size.

Allocate all load libraries using a block size of 32,760 bytes unless you plan to move your system software data sets from the device types on which they were originally allocated to device types with shorter track lengths, or plan to move them between device types having different track lengths without using IEBCOPY COPYMOD. A block size of 32,760 bytes will optimize space utilization and performance for all system software load libraries.