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SSDs: advantages

SSDs: advantages. exhibit higher speed than disks drive down power consumption offer standard interfaces like HDDs do. SSDs: critical technical constraints. the absence of in-place update the absence of random writing on pages

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SSDs: advantages

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  1. SSDs: advantages • exhibit higher speed than disks • drive down power consumption • offer standard interfaces like HDDs do

  2. SSDs: critical technical constraints • the absence of in-place update • the absence of random writing on pages • erasure limit : wear out after a certain number of program cycles

  3. Erasure limit: SLC vs MLC • SLC: 100,000 cycles • MLC: 10,000 cycles

  4. Erasure limit: RBER vs UBER

  5. Solution: SSD-RAID • RAID offers device-level redundancy • RAID is an effective method of constructing large-scale, high-performance, and high-reliability storage systems • SSD-RAID combines the advantages of the classic RAID and state-of-the-art SSDs

  6. Two parity-based SSD-RAID systems • Differential RAID • CSWL-RAID: Cross-SSD Wear-Leveling • They have a same assumption: parity blocks are updated more often than data blocks, and devices holding more parity receive more writes and consequently age faster

  7. Differential RAID • The Problem with RAID for SSDs: • they cause multiple SSDs to wear out at approximately the same rate

  8. Differential RAID: RAID5 case

  9. Differential RAID: features • Uneven Parity Distribution • Parity-Shifting Drive Replacement

  10. Uneven Parity Distribution: example • RAID-4: ( 100, 0, 0, 0, 0) • RAID-5: ( 20, 20, 20, 20, 20) • Diff-RAID: ( 40, 15, 15, 15, 15)

  11. Uneven Parity Distribution: aging rate

  12. Parity-Shifting Drive Replacement: example

  13. Parity-Shifting Drive Replacement: example

  14. Analysis of Age Distribution Convergence • Distribution of device ages at replacement time for (80,5,5,5,5) parity assignment

  15. Analysis of Age Distribution Convergence • Convergent distribution of ages at replacement time for different parity assignments

  16. Trade-off between reliability and throughput • the more skewed the parity distribution towards a single device • the higher the age differential • the higher the reliability • the lower throughput

  17. Diff-RAID Reliability Evaluation • Reliability of Diff-RAID • Reliability of Diff-RAID Configurations • Reliability with Different Flash Types • Reliability with Different ECC Levels • Reliability Beyond Erasure Limit • Reliability on Real Workloads

  18. Reliability of Diff-RAID • Diff-RAID reliability changes over time and converges to a steady state

  19. Reliability of Diff-RAID Configurations

  20. Reliability with Different Flash Types

  21. Reliability with Different ECC Levels

  22. Reliability Beyond Erasure Limit

  23. Reliability on Real Workloads

  24. Diff-RAID Performance Evaluation • Diff-RAID Throughput • Performance Under Real Workloads • Recovery Time

  25. Diff-RAID Throughput

  26. Performance Under Real Workloads

  27. Recovery Time

  28. Differential RAID: disadvantages • Assuming a perfectly random workload: without considering the actual age of devices • Parity-Shifting Drive Replacement: the procedure of reconstructing data and redistributing parity is complex and very time consuming • Trade-off between reliability and throughput: hard to determine a trade-off point

  29. CSWL-RAID: Why is CSWL needed • RAID5 and RAID6 cannot ensure wear leveling among devices under a imperfectly random workload

  30. CSWL-RAID: Basic Principle • change the wearing rate of some SSDs by dynamically adjusting the fraction of parity on them

  31. CSWL-RAID: Practical Architecture

  32. CSWL-RAID: Basic data layout Age distribution (1,1,1,1) Age distribution (3,3,3,1) Age distribution (2,2,1,1)

  33. CSWL-RAID: Improved data layout Age distribution (1,1,1,1) Age distribution (3,3,3,1) Age distribution (2,2,1,1)

  34. CSWL-RAID: Addressing Method RAID4 case Basic CSWL-RAID5 case RAID5 case

  35. CSWL-RAID: Addressing Method • Improved CSWL-RAID5 case

  36. CSWL-RAID: Addressing Method • Improved CSWL-RAID5 case

  37. CSWL-RAID: Average latency

  38. CSWL-RAID: Redistribution time CSWL-RAID5 case CSWL-RAID6 case

  39. CSWL-RAID: Age difference

  40. CSWL-RAID: Reliability

  41. CSWL-RAID: disadvantages • All SSDs wear out at approximately the same rate: lower reliability and shorter lifetime • Addressing method is too complex: the complexity of the addressing algorithm is O(t), where t denotes redistribution times

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