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 architecture Areal Density Quantity of bits within a track Disk RPM Disk access time T access = T seek + T rotate + T transfer T seek = t acc + t coast + t dec + t settle Power consumption Power = N platter x D platter 4.6 x RPM 2.8 Img src:

Current Design Limitations Disk access time Decrease t settle Basically remained constant Decrease D platter 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 reliability Increase 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 Nonvolatile Small physical size Lower power consumption High performance Used 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 endurance Memristors 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 Ge 2 Sb 2 Te 5 Fun fact: Discovery of semiconductor alloys along the GeTe- Sb 2 Te 3 line led to the 100GB cap Blu-ray disks

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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) T access = T seek + T rotate + T transfer = x = ms T two-layer = H cache x T cache + (1 - H cache )T access = x (1 -.65) = 2 ms

Hybrid (three-layer) disk access time T three-layer = H cache x T cache + (1 - H cache ) x (H flash x T flash + (1 – H flash ) x T access ) T three-layer (read) =.2 ms T three-layer (write) =.27 ms Compared to traditional disks 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 HDD Most 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 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 Future hard disk drive systems. Journal of Magnetism and Magnetic Materials 321 (2009),