Terminology

There is often confusion through the incorrect use of terminology when discussing the z/OS JCL SPACE operand, because people use the word "extent" when they mean "request" and the two are very different.

Difference in Terminology

A Request

This is what is made via the JCL DD statement SPACE operand, such as:

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

In the above example disk space of 15 whole cylinders is being requested, with the option to request additional lots of 5 cylinders if the amount of data exceeds the capacity of the initial 15.

An Extent

This is the area of a disk allocated to hold the data.

Requests and Extents are only equal if there is a sufficiently large area of free space to satisfy the request as a single entity.

If the system is unable to allocate the request, primary or secondary, in 1 extent, it will try to satisfy the request in up to 5 extents, before failing the task.