The volume mount analyzer does not know the physical length of a data set on a tape because the volume mount analyzer reads and analyzes SMF records, not the physical tape cartridges represented by the SMF records. Therefore, the volume mount analyzer data set size is the size of the data transfer to the tape cartridge.

The data set size calculation equals block count multiplied by block size. In most cases, the data transfer size equals the data set size. But since this calculation is an estimate, the two sizes are not always equal. If the data set is extended with DISP=MOD, the calculated size may be smaller than the actual size. If portions of the data set are reread or rewritten without closing and reopening the data set, the calculated size might be larger than the actual size. Examples are programs that reposition the tape with the BSP, POINT, CNTRL, EXCP, or CLOSE TYPE=T macros.

Since the type of tape cartridge, the compaction value, and the recording technique affect physical length of a data set on a tape, you need to be aware of the tape cartridge size, the compaction rate, and the recording technique.

To understand how data is placed on a tape, see z/OS DFSMS Using Magnetic Tapes.