Although small block sizes permit more concurrent channel operations, they reduce the net data transfer rate (the actual amount of data transferred per second). This can impact the elapsed time of a DFSORT application performing a significant amount of I/O. Small block size transfer also requires more CPU involvement and can, therefore, increase CPU time.

Large block sizes enable a higher net data transfer rate for sequential data sets, such as for input and output, and reduce the amount of processor time needed to service a channel program. For tapes, larger block sizes (up to 256 megabytes), provide the best performance and are recommended. Always use system determined blocksizes (SDB) for the best utilization and performance. An example of the benefits of appropriately large input and output data set block sizes is shown in Figure 1.

Figure 1. Benefits of Large Input/Output Data Set Block Sizes