The amount of space on a track or cylinder occupied by user data depends on the block size specified for the data set. Grouping logical records into blocks reduces the amount of space needed to store data. Because fewer physical records are needed to store the same number of logical records, the amount of space for count and key areas, and for gaps between records is reduced.

Larger block sizes offer better opportunities for increased disk space utilization. An appropriately selected block size can result in higher space utilization than a smaller block size. An inappropriately selected block size (large or small) can result in poor space utilization.

Figure 1 shows the 3390 space utilization with various block sizes for a fixed-blocked data set with a logical record length of 160.

Figure 1. 3390 Utilization for Various Block Sizes. Assumes data records are stored in equal-length physical records with no keys.