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SSD vs HDD - A Shift In Data Storage by Todd Dinkelman


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Originally presented by Todd Dinkelman, this presentation focuses on MLC drives. Specifically it covers:
􀂃 SSD Defined
􀂃 Quick Comparison
􀂃 Why MLC
􀂃 Reliability/Endurance
􀂃 Summary

Published in: Technology, Business

SSD vs HDD - A Shift In Data Storage by Todd Dinkelman

  1. 1. SSDs A Shift in Data Storage Todd Dinkelman Micron Technology, Inc. Santa Clara, CA USA August 2008 1
  2. 2. Agenda SSD Defined Quick Comparison Why MLC Reliability/Endurance Summary Santa Clara, CA USA August 2008 2
  3. 3. What is Being Called an SSD? USB interface embedded solution Module-based IDE interface card w/o enclosure 1.8-inch and 2.5-inch, 32GB and 64GB SSD for notebook and performance computing Module/card with custom form factor, density, and interface Santa Clara, CA USA August 2008 3
  4. 4. The Complexity of HDDs Cover mounting holes Base casting HDD Advantages • Density Spindle • Price/GB Slider (and head) Actuator arm Platters Actuator Flex circuit SATA interface (attaches heads connector to logic board) Power connector Actuator axis Source: Panther Products Santa Clara, CA USA August 2008 4
  5. 5. The Simplicity of SSDs SATA interface SSD Advantages connector Performance • Size • Weight • Ruggedness • Temperature • Range • Power NAND Printed Circuit Controller Santa Clara, CA USA August 2008 5
  6. 6. SSDs vs. HDDs Due to recent advances SSDs HDDs in NAND lithography, SSD densities have Capacity reached capacities for Performance mass market appeal Reliability SSDs offer many Endurance features that lead to improved user Power experiences Size Weight Early shortcomings regarding reliability and Shock endurance are being Temperature overcome Cost per bit Santa Clara, CA USA August 2008 6
  7. 7. SSDs in Computing CPU Relative Relative Latency Cost/Bit 1 L1 Cache 1,800 2.5 L2 Cache 1,400 1,200 DRAM 10 NAND 25,000 SSD 3 25,000,000 HDD 1 NAND Flash closes the latency gap Santa Clara, CA USA Cost/bit data as of November 2007 August 2008 7
  8. 8. NAND in Computer Architecture CPU DRAM Memory DRAM DDR3 PCIe Graphics & Bus Control Link Peripheral USB NVMHCI PCIe USB & Bus Flash Drive Flash Control SATA Hybrid SSD HDD Santa Clara, CA USA August 2008 8
  9. 9. MLC and SLC Differences SLC Single-level cell • – One bit per cell MLC Multi-level cell • – 2 bits per cell – today – 3 and 4 bits per cell – future Endurance SLC is typically 10 times better than MLC • Performance SLC provides ~2X the write performance of MLC • Price SLC-based products have better than 2X the • price/GB compared to MLC Santa Clara, CA USA August 2008 9
  10. 10. SSD Market Trends Improvements in controller technology • Moving from CompactFlash architectures to true SSD controllers Notebooks and PCs • Migration to MLC – Light usage model – Cost, size, and performance are all important • Value Proposition – Better than desktop performance in an ultra-light notebook Santa Clara, CA USA August 2008 10
  11. 11. SSD Average Price/GB 1.8quot; HDD MLC SSD $/GB 2004 2006 2008 2010 2012 2014 Santa Clara, CA USA August 2008 11
  12. 12. NAND Reliability and Endurance Effect Description Observed As Management Cells not being programmed Increased read Program ECC and Block receive charge errors immediately Disturb Management via elevated after programming voltage stress Cells not being Reliability read receive Increased read Read ECC and Block charge via error at high Management Disturb elevated voltage number of reads stress Data Charge loss over Increased read ECC and Block Retention time errors with time Management Cycles cause Failed Endurance/ Endurance charge trapped in program/erase Retire Block Cycling dielectric status Santa Clara, CA USA August 2008 12
  13. 13. NAND Error Rate Bit Error Rate – Failing bits corrected with appropriate levels of ECC – Correctable bit errors do not result in data loss Raw Bit Error Rate (RBER): Bit error rate prior to ECC Uncorrectable Bit Error Rate (UBER): Bit error rate after ECC UBER is projected using the measured RBER and specific level of ECC Santa Clara, CA USA August 2008 13
  14. 14. SSD Reliability and Endurance SSD reliability has two parts • MTBF – Measure of time between failures due to manufacturing or component defects – 2 million hours is typical for Micron SSDs • Endurance – SSDs all wear out due to data writes – Indication of drive life based on a usage condition – Micron SSDs are specified to last 5 years under predefined usage conditions – Usage conditions vary for consumer and performance products Santa Clara, CA USA August 2008 14
  15. 15. Endurance Factors Wear-leveling efficiency Write amplification NAND cycles SSD densities Santa Clara, CA USA August 2008 15
  16. 16. Wear-Leveling Dynamic wear-leveling uses the available free space and the incoming data to equally wear each of the physical blocks • Static data is not included in the available pool of wear-leveling blocks, leaving a portion of the drive with no wear Static wear-leveling considers all physical blocks in the SSD, regardless of content, and maintains an even level of wear across the entire drive Santa Clara, CA USA August 2008 16
  17. 17. Wear-Leveling Example MLC devices can typically support 10,000 cycles per block If you erased and reprogrammed one block every second, you would exceed the 10,000 cycling limit in just 3 hours! 60 x 60 x 3 = 10,080 Rather than cycling the same block, wear-leveling involves distributing the number of blocks that are cycled Santa Clara, CA USA August 2008 17
  18. 18. Wear-Leveling Example (continued) An 8GB MLC-based SSD contains 32,768 independent blocks (each block is 256KB of data) If we took the previous example and distributed the cycles over all 32,768 blocks, each block would have been programmed once after 9 hours If you provided perfect wear-leveling on an 8GB drive, you could erase and program a block every second, every day for over 10 years! 10,000 X 32,768 327,680,000 = = 3,792 days = 10.38 years 60 X 60 X 24 86,400 Santa Clara, CA USA August 2008 18
  19. 19. Write Amplification Additional write overhead due to garbage collection and wear-leveling • Write Amplification = Flash Writes / Host Writes Influence on host data patterns • Large number of small random transfers increases write amplification Influenced by number of spare blocks and static data • Keeping a percentage of the drive as spares will bound the write amplification Santa Clara, CA USA August 2008 19
  20. 20. Typical PC File Transfers Avg. Disk Bytes Per Read/Write 1800 1600 1400 1200 1000 800 Reads Writes 600 400 200 0 Less than 4KB to 16KB to 32KB to 64KB to 128KB to Greater 4KB 16KB 32KB 64KB 128KB 256KB than 256KB Transfer Size Santa Clara, CA USA August 2008 20
  21. 21. SSD Endurance Calculation Write amplification and wear-leveling efficiency must be accounted for when calculating SSD lifetime NAND Cycles * SSD Capacity Life in Years = Amplification Factor * GB/Year Santa Clara, CA USA August 2008 21
  22. 22. SSD Lifetime Given a capacity point, NAND endurance, and writes-per-day, the upper limit on SSD lifetime is defined Example: A 64GB SSD, 10K PROGRAM/ERASE cycles and write duty of • 350 GB/day will wear out in 5 years Uneven wear within a NAND Flash device may cause ‘hot spots’ Wear-leveling of NAND is necessary • Improper NAND data management may cause lifetime to be significantly less than the limit A Flash ERASE operation must occur before a page can be rewritten • To improve overall throughput, erase granularity is much larger than write • granularity LBA traffic pattern can cause lots of extra data movement • Santa Clara, CA USA August 2008 22
  23. 23. SSD Endurance Average Number Of Times Per Day The Drive Space Gets Written 60 33% WRITEs 50 2K random IOPs Times per Day (Count) 40 64GB drive (55GB user-accessible 30 20% reserved spares 20 10 0 0 12 24 36 48 60 72 84 96 Required Lifetime (Months) Santa Clara, CA USA August 2008 23
  24. 24. SSD Capacity Effect on Endurance Years for SSD to Wear Out Time to Failure 32 64 128 256 512 Drive Capacity (GB) SSD life will double with every doubling of capacity Santa Clara, CA USA August 2008 24
  25. 25. Summary SSDs bring many advantages to storage Performance • Power • Reliability • Environmental ruggedness • Different products for different markets • MLC-based SSDs for PCs Reliability has two aspects • Endurance and MTBF Santa Clara, CA USA August 2008 25
  26. 26. Thank You Santa Clara, CA USA August 2008 26