Delivering video files to streaming web servers is not the same as hosting a large database. I/O is done in an entirely different way, and knowing exactly how the workload interacts with the storage system is crucial to successfully size and design a robust storage solution.
How the workload defines requirements
Documentation
The application documentation is your best friend when trying to figure out what the storage requirements are. When there's an existing implementation of the application, ask the administrators what the software expects for performance and how it reacts when it doesn't meet the minimum requirements.