Each record written on direct access storage devices requires some device overhead.

Use the TRKCALC macro to calculate the exact number of bytes required for each data block including the space required for device overhead. For more information on how to use the TRKCALC macro, see z/OS DFSMSdfp Advanced Services.

If the TRKCALC macro cannot be used and space calculations must be performed manually, refer to the appropriate Direct Access Storage Reference Summary.

The following tables will help you estimate your space needs.

Table 1 lists the physical characteristics of DASDs. Today, disk storage subsystems other than those listed can emulate one of those listed. For example, the IBM® Enterprise Storage Server® (ESS) emulates the IBM 3390. On an emulated disk or on a VM minidisk, the number of cylinders per volume is a configuration option. It might be less than or greater than the stated number. If so, the number of bytes per device will differ accordingly. The IBM ESS Model 1750 supports up to 32760 cylinders, the IBM ESS Model 2107 supports up to 65520 cylinders and the IBM DS8000® supports up to 262668 cylinders.

IBM recommends letting the operating system choose an optimal block size for new data sets unless you have a reason to set a particular value. The system considers the data set type and other characteristics when choosing a block size as long as the data set does not have an undefined record format. Because the largest record supported by the access methods is a little less than 32 KB, the most efficient block size is not necessarily the maximum data length that can fit on the track.

For example, to maximize use of a 3380 track, 98.9% of the space available on a track can be used by writing two records of 23476 bytes each. The most efficient block size for the 3390 would be 27998 bytes; two of these blocks would fit on a 3390 track.

The maximum data length for a track multiplied by the number of tracks per cylinder produces the number of bytes available per cylinder for a device.

Similarly, the number of bytes per cylinder multiplied by the number of cylinders per volume produces the total number of bytes available for a device.