Agree, disk write usage wouldn't show capacity utilization. One needs to look at the file system utilization and make necessary changes (such as resize) to eliminate wasted space. Our experience is that most customers are way over provisioned from a capacity (and even performance point of view). Various opportunities such as right-sizing (manually doing it is a pain), right-typing (selecting gp2, st1 or sc1) io1 IOPS adjustments are available to significantly the reduce storage cost. Automating those tasks is key so that the solution can scale without user action.