By Szymon Jankowski

The Future of Disk Drives

Presentation Outline

Disk Drive Overview

Current Design Limitations

Proposed New Architecture

New Storage Media/Devices

Conclusion

Disk Drive Overview

Traditional architectureAreal Density

Quantity of bits within a trackDisk RPMDisk access time

Taccess = Tseek + Trotate + Ttransfer

Tseek = tacc + tcoast + tdec + tsettle

Power consumption Power = Nplatter x Dplatter

4.6 x RPM2.8

Img src: http://layout9.sohowebmaker.com/images/web_image_1088.jpg?pid=39372225011124490

Current Design Limitations

Disk access time Decrease tsettle

Basically remained constant Decrease Dplatter

Difficult to reduce smaller than 1.8 inches due to mechanical components and heat dissipation

Increase RPM Over 20K RPM, increased heat generation, power consumption,

noise, vibration and lack of long-term reliabilityIncrease AD

Cannot be scaled below 10nm due to superparamagnetic effect Superparamagnetic effect: smaller grain volume makes the

grains increasingly susceptible to thermal fluctuations, which decreases the signal sensed by the drive's read/write head

Proposed New Architectures

Dynamic Rotations Per Minute (DRPM) Constantly rotates platter at varying speeds Can only conserve energy for network servers

HDD with Multiple Spindles Two sets of heads and spindles, same chassis Reduced diameter platters

Reduce seek time and heat dissipation

Multiple disk actuators Second actuator is dedicated to reads, thus allowing

near-zero-access writes on the other head

New Storage Media/Devices

Media Flash memory Magnetic Random Access Memory (MRAM) Memristors Phase Change Memory

Devices Hybrid disk Solid State Disk

Media – Flash Memory

NonvolatileSmall physical sizeLower power consumptionHigh performanceUsed in systems where size and power or

performance are important (eg., smart phones, MP3 players, etc.)

NOR

NAND Faster erase/write times and higher data density Better candidate for data storage Accessed like block devices (eg., disk drives) Writes to free pages, written pages cannot be rewritten Garbage collection triggered when storage cap becomes

low Performance is normally very low during garbage collection

A block will wear out after between 10,000 to 1,000,000 program/erase cycles Can lead to shorter lifespan than that of hard disk

Media – MRAM, Memristors

MRAM Combines magnetic device with standard silicon-

based microelectronics Nonvolatile, high performance, fast programming,

unlimited endurance Random access, no refresh Expected to achieve the density of flash, except with

faster write speeds and unlimited enduranceMemristors

Remember amount of charge that has flowed through, even when turned off

Media – Phase Change Memory

PCM (or PCRAM) most closely resembles Dynamic Random Access Memory (DRAM)

Utilizes the large resistivity contrast between crystalline (low) and amorphous (high) phases of a chalcogenide Ge2Sb2Te5

Fun fact: Discovery of semiconductor alloys along the GeTe-Sb2Te3 line led to the 100GB cap Blu-ray disks

Img src: http://images.dailytech.com/nimage/4100_phasechange90nm.jpg

Uses large electric current to melt crystalline phase into amorphous phase

Medium current to harden amorphous into crystalline

Still in research phase

Devices – Hybrid Disk

Combines traditional disk with flash memory as a second-level cache

Stores “hot” items in the flash memory

Boots faster and saves energy

Traditional (two-layer) disk access time [Hitachi Ultrastar 15K147]

(using average values pulled from a datasheet) Taccess = Tseek + Trotate + Ttransfer

= 3.7 + 2 + 3.63 x 10-3

= 5.70363 ms

Ttwo-layer = Hcache x Tcache + (1 - Hcache)Taccess

= .65 + 6.4 x 10-4 + (1 - .65)5.70363 = 2 ms

Hybrid (three-layer) disk access time Tthree-layer = Hcache x Tcache + (1 - Hcache) x (Hflash x Tflash + (1 – Hflash) x

Taccess)

Tthree-layer(read) = .2 ms Tthree-layer(write) = .27 ms

Compared to traditional disk’s 2ms read/write time, hybrid disk is roughly 10 and 7.4 times faster, respectively

Devices – Solid State Disk

Semiconductor device used to emulate a HDDMost current SSDs use NAND flash memory

All the perks of NAND DRAM requires an internal battery and backup disk

Pros: Better random read performance Similar or better sequential read/write performance

Cons: Worse random write performance Still relatively expensive for capacity

Conclusion

MRAM, memristors and PCM are still being researched

Hybrid disk Required data may not be present in flash memory,

thus requiring the disk to spin up again High chance flash memory fails before magnetic disk Temporary solution

SSD Most likely candidate to replace HDD in next few

years Cost of NAND flash continues to decline while

capacity grows Faster than current mechanical disk drives Used in conjunction with disk drives on a server, could

save energy Less heat generated

Questions?

References

Chen, F., Koufaty, D., Zhang, X. Understanding Intrinsic Characteristics and System Implications of Flash Memory based Solid State Drives. The Ohio State University.

Deng, Y. 2011. What is the future of disk drives, death or rebirth? ACM Comput. Surv. 43, 3, Article 23 (April 2011), 27 pages.

Wong, P., Raoux, S., Kim, S., Liang, J., Reifenberg, J., Rajendran, B., Asheghi, M., Goodson, K. Phase Change Memory. Stanford.edu.

Wood, R. 2008. Future hard disk drive systems. Journal of Magnetism and Magnetic Materials 321 (2009), 555-561.