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Storage Alternatives for Mobile Computers

Storage Alternatives for Mobile Computers. 2008 년 09 월 23 일. 정보컴퓨터과학과 컴퓨터과학전공 72071834 김진성. Table of Contents. Overview Mobile storage subsystem Hardware Measurements Trace-Driven Simulation Result Conclusion. Read. Write. Read. Write. Read. Write. 1. Overview.

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Storage Alternatives for Mobile Computers

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  1. Storage Alternatives for Mobile Computers 2008년 09월 23일 정보컴퓨터과학과 컴퓨터과학전공 72071834 김진성

  2. Table of Contents • Overview • Mobile storage subsystem • Hardware Measurements • Trace-Driven Simulation • Result • Conclusion

  3. Read Write Read Write Read Write

  4. 1. Overview • Mobile Computers require low weight, low power consumption, and good interactive performance • Choices for Mobile storage subsystem • Magnetic disk(hign energy consumption, spin-up delays) • Flash disk emulator(expensive, poor throughput) • Flash memory card(expensive, fragmentation effects) • Examine trade-offs • Energy consumption • Performance • Cost-capacity • Evaluation via measurements and simulation • Flash saves energy, offers good performance • Gap between flash card theory and practice

  5. 2. Mobile Storage Subsystem 2.1 Mobile storage subsystem • Magnetic Hard Disk • Large capacity • Lowest cost • High throughput for large transfers • A lot of energy • Spin-up and down • Flash Memory • Relatively little energy • Low latency • High throughput for read accesses • Costs expensive more than ($30~50/Mbyte, hard Disk $1~5/Mbyte) • Require erasing before it can be overwritten

  6. 2. Mobile Storage Subsystem 2.1 Mobile storage subsystem • Flash Memory Card • Large capacity • Lowest cost • High throughput for large transfers • A lot of energy • Spin-up and down

  7. 2. Mobile Storage Subsystem 2.1 Mobile storage subsystem • Flash memory card Vs Flash disk emulators • Flash memory cards accessed as main memory • Flash disk emulators accessed through a disk block interface • These devices behave differently, having varying access times and bandwidth

  8. ※ Experimental Methodology • Comparisons on H.P. OmniBook • Small devices limit experiments • Peak performance but not actual usage patterns • Trace-driven simulation of larger devices • Simulate real workloads • Vary policies • Evaluation of flash card storage utilization • Fill up flash card on Omnibook • Real workloads with varying utilization • Simulate newer flash disk emulator

  9. 3. Hardware Measurements • Comparisons on H.P. Omnibook • Small devices limit experiments • Peak performance but not actual usage patterns • H.P. OmniBook 300 • 2.9 pound Nootbook Computer • MS-Dos 5.0 • 25MHz 386SXLV Processor • 2Mbytes DRAM

  10. 3. Hardware Measurements

  11. 3. Hardware Measurements

  12. 3. Hardware Measurements

  13. 3. Hardware Measurements

  14. 3. Hardware Measurements

  15. 4. Trace-Driven Simulation • Taken from 3 platforms • Macintosh PowerBook Duo (Bill Sproule, MITL) • IBM PCs running Windows 3.1 (Kester Li, U.C. Berkeley) • HP workstation running HP-UX (Ruemmler & Wilkes, HP) • Dataset sizes 16-32Mbytes • Trace duration 2hrs-4days • Read/Write ratio 25-50% • Mean interarrival time

  16. 4. Trace-Driven Simulation

  17. 5. Result 5.1 Basic Comparisons

  18. 5. Result 5.2 Flash Storage Utilization

  19. 5. Result 5.2 Flash Storage Utilization

  20. 5. Result 5.2 Flash Storage Utilization

  21. 5. Result 5.2 Flash Storage Utilization • When flash is full, how to erase blocks? • Flash disk: small erasure units no effect • Flash card: fragmentation becomes issue • Similar to segment cleaning in over-utilized LFS • High utilization impacts energy, write latency, and durability

  22. 5. Result 5.2 Flash Storage Utilization

  23. 5. Result 5.2 Flash Storage Utilization

  24. 5. Result 5.2 Flash Storage Utilization

  25. 5. Result 5.2 Flash Storage Utilization ※ Flash Card Utilization

  26. 5. Result 5.3 Asyhchronous Cleaning • SunDisk SDP5A allows asynchronous erasure • Have choice of erasing in advance or combined operation • Erasing by itself takes same time as combined erase/write (150KB/s) • Writing pre-erased blocks go at 400KB/s • Can erasure be effectively decoupled from write? • Minimal difference in energy consumption, but average write latency drops 55-60% • Requires system to think of device as flash, not disk

  27. 5. Result 5.4 DRAM Caching

  28. 6. Conclusion • Flash compares favorably to magnetic disks • Energy: roughly an order of magnitude less than Caviar • Read latency up to two orders of magnitude faster • Write latency configuration issue • Cost-capacity still poor, but acceptable • Flash cards suffer from erasure unit • Significant degradation above 80% utilization • Complexity of cleaning (c.f. Microsoft FFS 2.0) • Surprising gap between theory and practice • SRAM essential for hiding write latency • More common for magnetic disks than flash so far • Watch technology trends

  29. Q&A

  30. Thank You!

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