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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.