Capacity



On older 9-track tape, up to and including 3480s, the IBG was between 0.3 and 0.6 inches depending upon model.

For 3490s and above the IBGs are managed by the unit which combines multiple logical blocks into one larger physical block which reduces, but does not eliminate the issue.

Disk IBGs have always been measured in bytes rather than inches.

The next four demonstrations illustrate the effect of IBGs on logical capacity in relation to 3390 emulated disks.

Logical versus Physical capacity



This is the area of debate, and the rationale for this tutorial.

The actual disks within the Direct Access Storage Device (DASD) Sub-system are not 3390 Count Key Data (CKD) devices, so the back end storage (physical capacity) is no longer directly governed by the JCL SPACE operand.

The important thing to remember is that z/OS is unaware of the emulation and therefore makes its calculation on how much data (logical capacity) can be accommodated within a given quantity of SPACE.

That decision will be influenced by the size of the data blocks being written, and the number of IBGs required, with an Sx37 (End-of-Volume (EOV)) ABEND being the result if insufficient space has been assigned.

The following four demonstrations, which are executable on your own system, are intended to illustrate that z/OS is making decisions as though it were communicating with real 3390 volumes.

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