Disk Drive Roadmap from the Thermal Perspective A Case for Dynamic Thermal Management

1. Disk Drive Roadmap from the Thermal PerspectiveA Case for Dynamic Thermal Management Sudhanva Gurumurthi Anand Sivasubramaniam, Vivek Natarajan Computer Systems Lab Pennsylvania State University

2. Power Demands of Data Centers“What matters most to the computer designers at Google is not speed but power – low-power – because data centers can consume as much electricity as a city”, Eric Schmidt, CEO, Google • Data centers consume several Megawatts of power • Electricity bill • $4 billion/year • Disks account for 27% of computing-load costs • Difficult to cool at high power-densities Sources: 1. “Intel’s Huge Bet Turns Iffy”, New York Times article, September 29, 2002 2. “Power, Heat, and Sledgehammer, Apr. 2002. 3. “Heat Density Trends in Data Processing, Computer Systems, and Telecommunications Equipment”, 2000.

3. Data Center Cooling Costs • Data center of a large financial institution in New York City • Power consumption ~ 4.8 MW Source: “Energy Benchmarking and Case Study – NY Data Center No. 2”, Lawrence Berkeley National Lab, July 2003.

4. Temperature Affects Disk Drive Reliability • Heat-Related Problems • Thermal-tilt of disk stack and actuator arms • Out-gassing of spindle/voice-coil motor lubricants • Wear-out of bearings • Hard disk operating 5 C above normal temperature 10-15% more likely to fail • Disk drive design constrained by the thermal-envelope

5. Source: Hitachi GST Technology Overview Charts, http://www.hitachigst.com/hdd/technolo/overview/storagetechchart.html

6. Data-Rate Capacity Increase RPM Shrink Platter (Dia)4.6 (RPM)2.8 (# Platters) Temperature Thermal-Constrained Design Data Rate =~ (Linear-Density)*(RPM)*(Diameter) 1 platter Can we stay on this roadmap? Lower Capacity Lower Data Rate 40% Annual IDR Growth Increase RPM Power =~(# Platters)*(RPM)2.8*(Diameter)4.6

7. Outline • Introduction • Modeling • The Roadmap • Dynamic Thermal Management • Conclusions

8. Modeling • Baseline input parameters • Linear-Density (BPI) • Track-Density (TPI) • Characteristics Modeled • Capacity • Performance • Temperature

9. Capacity Model • Cmax = ηxnsurfxπ(ro2-ri2)(BPIxTPI) • Stroke-Efficiency:η < 1 • Spare tracks, recalibration tracks etc. • Assumed η = 2/3 [CMRR] • User-accessible capacity needs to be derated due to: • Zoned-Bit Recording (ZBR) • Servo Overheads • ECC Overheads

10. Performance Models • Parameters Modeled • IDR • Seek-time • IDR • IDR experienced by outermost zone • Seek-time • Uses linear-interpolation based on track-to-track, average, and full-stroke times [Worthington’95] • Accurate for seeks longer than 10 cylinders

11. Validation • Compared modeled vs. actual capacity and IDR using 13 disks from 4 different manufacturers from 1999-2002 • Inputs: BPI, TPI, RPM, Platter-size, Number of platters • Assumed all disks have 30 zones.

12. Performance Model Validation

13. Source: Hitachi GST Technology Overview Charts, http://www.hitachigst.com/hdd/technolo/overview/storagetechchart.html

14. Change in BPI and TPI Trends • Slowdown in BPI • Difficult to lower fly-height • Requires higher recording media coercivity • Smaller grain sizes suffer from superparamagnetic effects • Slowdown in TPI • Narrower tracks more susceptible to media noise • Inter-track interference • Increase in track edge-effects with narrower tracks • Bit-Aspect Ratios (BPI/TPI) dropping • Larger slowdown in BPI • Long-term areal density growth expected to slowdown to 40-50% • 1 Tb/in2 disk expected to be available in 2010 [DS2]

15. Capturing BPI and TPI Trends • Studied published work on designing Terabit areal-density disks. • Chose design with most conservative assumptions about BPI • Scaled BPI and TPI CGRs to achieve 1 Tb/in2 areal density in 2010 • BPI CGR = 14% • TPI CGR = 28% • Areal-density CGR = 46%

16. Thermal Model • Extension of work by Eibeck et al. at the University of California • Components Modeled: • Internal air • Spindle-assembly • Arm-assembly • Drive base and cover • Drive completely enclosed • External temperature maintained constant

17. Modeling the Heat-Transfer • Newton’s Law of Cooling: dQ/dt = hAΔT • Internal Air Heat = Heat convected by solid components + viscous dissipation – heat lost through drive cover

18. Drive Parameters • Materials • Proprietary data • Assumed platters, arms, and spindle-hub composed of Aluminum • Geometry • Modeling and measurement • Voice-coil motor (VCM) power • Used published data from IBM [Sri-Jayantha’95] • External air temperature • Assumed 28 C for single-platter configuration

19. The Thermal-Envelope Thermal Envelope

20. Outline • Introduction • Modeling • Formulating a Disk-Drive Roadmap • The Roadmap • Dynamic Thermal Management • Conclusions

21. Drive RPM Areal Density ≥ 1 Tb/in2 BPI CGR = 30% TPI CGR = 50% BPI CGR = 14% TPI CGR = 28%

22. Drive Temperature Thermal-Envelope

25. Outline • Introduction • Modeling • Formulating a Disk-Drive Roadmap • The Roadmap • Dynamic Thermal Management • Conclusions

26. Dynamic Thermal Management (DTM) • To boost performance while still working within the thermal-envelope by dynamic activity-control • How much do higher RPMs benefit application I/O performance?

27. Applications Studied • Five commercial I/O traces • Openmail (HP Labs) • OLTP Application (UMass Repository) • Web Search-Engine (UMass Repository) • TPC-C (Penn State) • TPC-H (IBM Research) • Attempted to re-create the disk-system on which the trace was collected in DiskSim

28. 30-60% Performance Boost for 10,000 RPM Increase

29. Search-Engine - Thermal BehaviorThermal Envelope = 45.22 C

30. SPM+VCM On Thermal Slack RPM VCM Off DTM Solution 1:Exploiting Thermal Slack T E M P E R A T U R E Thermal-Envelope TIME

31. Thermal Slack

33. DTM Solution 2:Activity Throttling • Thermal-design assuming an average-case operation • Basic idea • Disk services requests at its peak-performance configuration • Throttle disk activities if thermal-envelope may be exceeded

34. VCM On VCM Off Approach 1:Seek Throttling T E M P E R A T U R E Thermal-Envelope TIME

35. VCM On VCM Off VCM Off+ Low RPM Approach 2:(Seek+RPM) Throttling T E M P E R A T U R E Thermal-Envelope TIME

36. Throttling-Ratio 2.6” 40% IDR Growth to 2005 2.6” 40% IDR Growth to 2007 • tcool – Disk undergoing throttling • theat – Disk operating at maximal performance configuration • Throttling-Ratio = (theat/tcool)

37. Summary • Need aggressive RPM increases to sustain IDR growth • Scaling BPI and TPI more difficult • Lower Signal-to-Noise ratios at higher densities increase ECC overheads • IDR growth would get affected due to heat dissipation • 40% growth rate cannot be maintained beyond 2007 even for 1.6” platter-size • Expected to slowdown to 14% • Possible to buy back performance with Dynamic Thermal Management (DTM).