Direct access storage devices

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 emulate the track capacity of a IBM 3380 or 3390 device while providing much larger capacity than the original 3380 and 3390 devices. For example, the IBM System Storage DS8000 series emulates 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 2105 supports up to 65520 cylinders and the IBM DS8000® supports up to 1,182,006 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 (RECFM=U) 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. Start of changeHowever, in some cases, you must consider data set type when determining the most efficient block size. For example, extended format data sets have a 32-byte suffix for each block and encrypted basic and large format data sets have an 8-byte prefix for each block. In these cases, the most efficient block size must be adjusted to take into account the length of the suffix or prefix.End of change

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.

Table 1. DASD Physical Characteristics
Type Most Efficient Block Size Maximum Data Length/Track Trk/Cyl Bytes/Cyl Avail for User Records Cyl/Vol
3390 emulation using IBM DS8000 device 27,9981 56,664 15 849,960 1 – 1,182,006
3380 emulation using IBM DS8000 device 23,4761 47,476 15 712,140 1 – 1,182,006
3390 Model 1 27,9981 56,664 15 849,960 1,113
3390 Model 3 27,9981 56,664 15 849,960 3,339
3390 Model 9 27,9981 56,664 15 849,960 10,017
9345 Model 1 22,9281 46,456 15 696,840 1,440
9345 Model 2 22,9281 46,456 15 696,840 2,156
Note:
  1. Two-record format