Their are two commands RTVDSKINF and PRTDSKINF which collect and print disk space usage information. These are part of the operating system. You should probably run this under a user profile name that has *ALL authority. This will show you how much space each library on the system takes up, as well as other system information such as space taken up by repors still on the system, folders and documents, etc.
The data is collected in the RTVDSKINF command into a file named QAEZDISK if you take the device parameter default of *SYSBAS or QAEZDnnnnn if you collect data for a particular ASP where nnnnn is the ASP number. The PRTDSKINF command has a parameter that will allow different types of reports to be printed.
For simple DASD usage history, consider the Retrieve System Status (QWCRSSTS) API with format SSTS0200. The storage capacity of the system auxiliary storage pool, percentage used and other info can be collected in a single call. Run your program once a week and insert values into a table. Over time, you’ll have a clean, neat table showing the trend.
Whenever spikes appear, you can check the QAEZDISK data that should be collected separately, perhaps also weekly.