Charging for cloud storage must account for two costs: the cost of the capacity used and the cost of access to that capacity. For the cost of access, current systems focus on the work requested, such as data transferred or I/O operations completed, rather than the exertion (i.e., effort/resources expended) to complete that work. But, the provider’s cost is based on the exertion, and the exertion for a given amount of work can vary dramatically based on characteristics of the workload, making current charging models unfair to tenants, provider, or both. This paper argues for exertion-based metrics, such as disk time, for the access cost component of cloud storage billing. It also discusses challenges in supporting fair and predictable exertion accounting, such as significant inter-workload interference effects for storage access, and a performance insulation approach to addressing them. Acknowledgements: We thank the members and companies of the PDL Consortium (including APC, EMC, Facebook, Google, HP, Hitachi, IBM, Intel, LSI, Microsoft, NEC, NetApp, Oracle, Riverbed, Samsung, Seagate, STEC, Symantec, VMware, and Yahoo!) for their interest, insights, feedback, and support. We also thank Intel and NetApp for hardware donations that enabled this work. This material is based on research sponsored in part by the National Science Foundation, via grants #CNS-0326453, #CCF-0621499, and #CNS-0917012, by the Department of Energy, under award number DE-FC02-06ER25767, by CyLab at Carnegie Mellon, under agreement number DAAD19-12-1-0389 from the Army Research Office and by an NDSEG Fellowship sponsored by the Department of Defense.