"Disk time is what costs, not I/Os or bytes, and that is what should be the metric in cloud storage systems," said Matthew Wachs, a researcher at Carnegie Mellon University, in a talk at the Usenix HotCloud workshop this week in Portland, Oregon.

Wachs, along with other researchers at Carnegie Mellon and VMware, investigated the topic in their Usenix paper, "Exertion-based billing for cloud storage access."

"Cloud storage access billing should be exertion-based, charging tenants for the costs actually induced by their I/O activities rather than an inaccurate proxy (e.g., byte or I/O count) for those costs," the paper said.

Today, IaaS (Infrastructure-as-a-Service) cloud storage providers such as Amazon or Google typically bill on two factors, the amount of data being stored and the amount of data that is transferred to and from the cloud, or I/O.