An extent is a contiguous area of disk space with an absolute starting location on the disk device and an absolute ending location on that same disk device.
A free extent is a contiguous area of disk space with an absolute starting location on the disk device and an absolute ending location on that same disk device that is available for any new primary and secondary extent allocation sizes that will fit.
A few simple diagrams to visualize z/OS disk space allocation follow:
A large enterprise has thousands of disk devices, each with a unique disk device address
The disk storage administrator or systems programmer use a system utility to initialize disk volumes,
allocating a label and a VTOC, Volume Table of Content.
The label points to the disk location of the VTOC.
The VTOC has starting and ending location of every extent (primary, secondary, and free extents).
Here's a list of each type of data set and the max number of extents for each type:
|Data Set Type||Max Extents|
|Sequential||16 total = 1 primary + 15 secondary|
|Sequential Extended||123 total = 1 primary + 122 secondary|
|PDS||16 total = 1 primary + 15 secondary|
|PDS/E||123 total = 1 primary + 122 secondary|
|VSAM||255 total = 1 primary + 254 secondary|
|VSAM (extent constraint removed)||n+1 total = 1 primary + n secondary|
So, how does one know where an individual data set extent begins and ends?
The answer is that each disk device has a Volume Table of Contents (VTOC). The VTOC is a single extent written to the disk device when first initialized and formatted. Without any further changes to the VTOC, the disk has one large free extent that covers the entire device. As data sets are allocated to the disk, the VTOC is amended to indicate where each extent starts and stops. Conceptually, the VTOC is similar to the File Allocation Table (FAT) in some PC disks.
Table of disk device capacity for record storage