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