Figure 1 shows the relationship between control interval size, physical block size, and track capacity that is not in extended format.

Figure 1. Control Interval Size, Physical Track Size, and Track Capacity

The information about a track is divided into physical blocks. Control interval size must be a whole number of physical blocks. Control intervals can span tracks. However, poor performance results if a control interval spans a cylinder boundary, because the read/write head must move between cylinders.

The physical block size is always selected by VSAM. VSAM chooses the largest physical block size that exactly divides into the control interval size. The block size is also based on device characteristics.