Clean disk scenario

In this scenario there are large areas of free space, so each request for space can be satisfied with 1 extent as indicated opposite.

Scenario 1 - Either Empty Disk or one with large free areas.

As on an earlier page the request for disk space was:

//ddname DD SPACE=(CYL,(15,5))

Give the large amount of free space, if the amount of data exceeds the capacity of the initial request of 15 cylinders, then the potential disk space available could be:

Request	Extents	Allocation
15	1	15 Cylinders satisfying Primary
5	15	75 (5 * 15) Cylinders satisfying Secondary
	----	---------------------------------
	16	90 Cylinders across 16 extents

The secondary request amount is saved in the F1 DSCB, allowing the value to be used to extend the data set at a later time.

All to often this idealistic scenario is not available, so let us look at a possible worst case.