Magnetic tape units

The following table describes the maximum block size that is supported by BSAM and QSAM on magnetic tape devices (in bytes).

Tape	Without LBI	Optimum with LBI¹	Maximum with LBI
3420 Magnetic Tape Units (7 track and 9 track)	32,760	32,760	32,760
3430 Magnetic Tape Units	32,760	32,760	32,760
3480 Magnetic Tape Subsystem (with or without compaction mode)	32,760	65,535	65,535
3490 and 3490E Magnetic Tape Subsystem (with or without compaction mode)	32,760	65,535	65,535
3590	32,760	229,376 or 262,144	262,144

Note:

LBI stands for large block interface. You can request it by coding the BLKSIZE keyword on the DCBE macro or turning on the DCBEULBI bit before completion of the DCB OPEN exit routine. If the system accepts the request, OPEN turns on the DCBESLBI in the DCBE after the DCB OPEN exit routine. This bit affects several BSAM and QSAM interfaces. Refer toz/OS DFSMS Using Data Sets.