DRPM: Dynamic Speed Control for Power Management in Server Class Disksby: Gurumurthi et. al.presented by: James Larkby-LahetMotivation•22.3 W IBM Ultrastar 36ZX server disk vs. 57.8 W 1.6 GHz Intel Xeon CPU•4-way Xeon SMP with 140 disks used by Dell for TPC benchmark uses 13.5 times more power for disks than CPUs•current approaches•on - no perf compromise or power saved•spindown - savings, but serious penaltyDRPM vs TPM•Spindle motor is 50-80% of idle power•for short idle periods TPM may expend more energy than is saved•DRPM doesn’t have long spin up delay and can service requests in a lower power (speed) mode (with longer rotation and transfer delays)•DRPM allows adaptive tradeoff between power and performanceSpindle Motors•permanent magnet DC brushless motor•speed is achieved by ‘pulsing’ the motor power, it is not always on•sensors provide speed feedback•high torque needed to start platter spinningPower Model•4 methods used to determine relationship between RPMs and power•plot of multi-generational devices•physics•Sony Multimode Hard-disk•static configuration•not enterprise - 2 rather than 10 platters•IBM paper on disk designMulti-generation IBM DisksPhysicsSony Multimode DiskIBM Disk Design StudyTransition TimesOracle ComparisonPlatter CountPower ModelDRPM Heuristic•watermark is set by array•periodically, disk checks its queue length, if 0, moves to next lowest state unless at watermark10-15% vs 5-8%10-15% vs 5-8%10-15% vs 5-8%Issues•Providing Speed Control - Pulse Width Modulation•Fly-Height - Papillon slider is constant over DRPM range•Head Positioning - servo sectors: may not see enough at low speed, but this is solved•Idle-Time activities - DRPM good for QoS•Reliability - DRPM may reduce? though avoids spin down duty cyclingConclusion•DRPM recovery is faster and takes less energy than spinup•able to exploit short idle periods•can provide a reduced service level•orthogonal to spindown•simple heuristic can save more power than oracle (DRPM or
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