Hi Jenny,
There is an OOTB report that gives you some of this detail already. Here is the navigation for it:
IT Management - Applications OR Apps/Services - App TCO
Click on the By IT Tower tab
Find Storage in the table or in the bar chart and click on it (drill down)
This navigates you to a drill down of each storage sub-tower by application:

You can either use this instead of building a new report or use this as the basis for a new report.
Hope this helps,
Lauren Griessel