The use of tape work storage devices prevents the use of the more efficient Blockset technique. Best performance, using tape intermediate storage, is usually obtained when you use six or more tape drives of the fastest type. As a general rule, you should use as many tapes as you have available for intermediate storage. A larger number of tapes increases the number of strings that can be merged in one pass, and, therefore, decreases the number of passes required in the intermediate merge phase. This then reduces elapsed time and, often, the number of I/O operations.

Increasing the number of work units, however, also reduces the block size used for intermediate storage; this can become a critical factor if you have relatively little main storage available for buffers. For example, if DFSORT has only 88KB in which to operate, you probably achieve no improvement (and might find deterioration) if you use more than four tape work units. Therefore, apply the general rule of using as many tapes as possible only when DFSORT has more than 100KB available.

For information on how to determine storage requirements when using different tape techniques, see Using work space.