WRKSYSSTS and WRKDSKSTS are 2 of the most useful commands for disk space. It might also be worth looking at ops navigator. The ops navigator can run some tools for collecting data over a period of time.
If basic tracking is desired, the <a href=”http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=/apis/qwcrssts.htm”>Retrieve System Status (QWCRSSTS) API</a> can be called on a daily (or more often) schedule. The returned values could be written to a “system status” log file. This could be kept for a long time to show growth trends or other information. One record per day means the file could continue for a long time.