Analysis of disk power management for data-center storage systems

Up to one third of the electricity supplied to a data center is required for the operation of the storage subsystem. A typical data-center workload, characterized by short idle periods, prevents traditional disk power management (DPM) from saving energy. This paper starts with an analysis of DPM's traditional timer-based disk spin-down policy. Next, we examine how a multispeed disk adapts DPM to datacenter workloads. Finally, we determine how to shape the workload to enable DPM on conventional server disks. All analysis is based on an analytical model for the energy consumed by a disk during an idle period. In addition, we assume standby power to be non-negligible. The results are that (1) the competitive ratio of a threshold-based disk spin-down policy depends on the ratio of standby power to idle power, (2) the notion of break-even time can be generalized for multispeed disks, and (3) DPM saves most energy when mean idle time and idle-time variance are maximized. With this analysis, the authors intend to stimulate the design of new data-center file and storage systems that optimally exploit DPM to save energy.