Calculating Space for the Data Component of a KSDS

You can use the following formula for any DASD. The number of blocks per track and control intervals per track depends on the DASD you are using. The following example shows how to calculate the size of the data component for a key-sequenced data set. The following are assumed for the calculations:

Device type. 3390
Unit of space allocation. Cylinders
Data control interval size. 1024 bytes
Physical block size (calculated by VSAM). 1024 bytes
Record size. 200 bytes
Free space definition – control interval. 20%
Free space definition – control area. 10%
Number of records to be loaded. 3000

You can calculate space for the data component as follows:

Number of bytes of free space (20% × 1024) = 204 (round down)
Number of loaded records per control interval (1024–10–204)/200 = 4.
Number of physical blocks per track = 33.
Number of control intervals per track = 33.
Maximum number of control intervals per control area (33 x 15) = 495.
Number of loaded control intervals per control area (495 - 10% x 495) = 446.
Number of loaded records per cylinder (4 x 446) = 1784.
Total space for data component (3000/1784) (rounded) = 2 cylinders.

The value (1024 – 10) is the control interval length minus 10 bytes for two RDFs and one CIDF. The record size is 200 bytes. On an IBM 3380, 31 physical blocks with 1024 bytes can be stored on one track. The value (33 × 15) is the number of physical blocks per track multiplied by the number of data tracks per cylinder.