Why disk write usage? Assuming the reason for analysis is cost, storage space utilization is more important. GP2 and other non-provisioned IOPS types are priced per GB. I've seen EBS costs amount to 50% of the AWS bill at a couple companies, so reducing disk size can be a good cost saver[1]. You need an agent/script running in the OS to get disk space utilization.
[1] As another poster pointed out, sometimes you want to over provision size to get IOPS.