隨著儲存裝置的技術發展，快閃記憶體所組成的固態硬碟已逐漸增加並成為主要儲存工具。為了要增加快閃記憶體的單位儲存量，新的多階儲存單元 (MLC)的快閃記憶體技術被提出，它不但能增加快閃記憶體的儲存量，還能降低快閃記憶體的生產成本。然而，此技術卻造成快閃記憶體的寫入效能明顯降低。本篇論文提出一個可選擇性跳躍方法(SSKIP)去改善MLC快閃記憶體固態硬碟的寫入效能。SSKIP可以掌握MLC快閃記憶體中不同頁面的寫入效能差異，它會跳過寫入效能較差的頁面並選擇寫入效能較佳的頁面去存放資料。SSKIP也考慮跳躍頁面所造成垃圾回收次數上升的影響，它可以避免因為垃圾回收程序所產生的固態硬碟效能降低，確保跳躍頁面可以有效改善固態硬碟效能。此外，SSKIP也會預防跳躍頁面所引起的快閃記憶體壽命問題，它利用配額去限制固態硬碟中每秒所增加的占據空間，並根據此配額決定是否執行跳躍動作。
我們採用從真實系統中所產生的儲存裝置存取行為去評估SSKIP的效能，結果顯示SSKIP不僅能夠有效改善固態硬碟的平均反應時間，同時也可以確保固態硬碟可以正常運作至指定年限。

With the development of storage technology, NAND flash memory-memory based solid-state drives (SSDs) have been increasingly used in storage systems. In order to increase the bit density in NAND flash memory, a multi-level cell (MLC) NAND flash memory is proposed to enable higher capacities and lower prices, but these improvements bring about higher flash program latencies on each write request. In this paper, a selective skip mothed called SSKIP is proposed to improve the MLC write performance. SSKIP is aware of the performance variability in MLC NAND flash memory. It skips the pages with slower speed and directs data to the pages with higher speed. SSKIP also considers the possible overhead of garbage collection for each skip operation. It can avoid the influence of garbage collection effectively, so the performance improvement of skip operation is not reduced drastically. Moreover, SSKIP prevents the lifetime crisis caused by skip operations. It judges the frequency of skip according to the occupied size per second. We shows the effectiveness of SSKIP by experiments on a set of workloads gathered from real world. The experiment results shows that our SSKIP can achieve a significant reduction in average response time and the lifetime of SSD is not in danger.

[1]L. M. Grupp, A. M. Caulfield, J. Coburn, S. Swanson, E. Yaakobi, P. H. Siegel, et al., "Characterizing flash memory: anomalies, observations, and applications," in Microarchitecture, 2009. MICRO-42. 42nd Annual IEEE/ACM International Symposium on, 2009, pp. 24-33.

[2]X. Jimenez, D. Novo, and P. Ienne, "Libra: Software-Controlled Cell Bit-Density to Balance Wear in NAND Flash," ACM Transactions on Embedded Computing Systems (TECS), vol. 14, p. 28, 2015.

[3] J. Lee and D. Shin, "Adaptive Paired Page Prebackup Scheme for MLC NAND Flash Memory," Computer-Aided Design of Integrated Circuits and Systems, IEEE Transactions on, vol. 33, pp. 1110-1114, 2014.

[4] L. M. Grupp, J. D. Davis, and S. Swanson, "The Harey Tortoise: Managing Heterogeneous Write Performance in SSDs," in USENIX Annual Technical Conference, 2013, pp. 79-90.

[5]D.-W. Chang, H.-H. Chen, D.-J. Yang, and H.-P. Chang, "BLAS: Block-level adaptive striping for solid-state drives," ACM Transactions on Design Automation of Electronic Systems (TODAES), vol. 19, p. 21, 2014.

[6]A. Gupta, Y. Kim, and B. Urgaonkar, DFTL: a flash translation layer employing demand-based selective caching of page-level address mappings vol. 44: ACM, 2009.

[7]A. M. Caulfield, L. M. Grupp, and S. Swanson, "Gordon: using flash memory to build fast, power-efficient clusters for data-intensive applications," ACM Sigplan Notices, vol. 44, pp. 217-228, 2009.

[8]S.-H. Park, J.-W. Park, J.-M. Jeong, J.-H. Kim, and S.-D. Kim, "A mixed flash translation layer structure for SLC-MLC combined flash memory system," in Proceedings of the 1th International Workshop on Storage and I/O Virtualization, Performance, Energy, Evaluation and Dependability (SPEED2008), 2008.

[9]S. Im and D. Shin, "Storage architecture and software support for SLC/MLC combined flash memory," in Proceedings of the 2009 ACM symposium on Applied Computing, 2009, pp. 1664-1669.

[10]L.-P. Chang, "Hybrid solid-state disks: combining heterogeneous NAND flash in large SSDs," in Design Automation Conference, 2008. ASPDAC 2008. Asia and South Pacific, 2008, pp. 428-433.

[11]S. Im and D. Shin, "ComboFTL: Improving performance and lifespan of MLC flash memory using SLC flash buffer," Journal of Systems Architecture, vol. 56, pp. 641-653, 2010.

[12] S. Lee, K. Ha, K. Zhang, J. Kim, and J. Kim, "FlexFS: A Flexible Flash File System for MLC NAND Flash Memory," in USENIX Annual Technical Conference, 2009, pp. 1-14.

[13]Silicon Graphics International, "Solid State Disk Solutions". Available: http://www.sgi.com/products/storage/infrastructure/ssd.html?/endurance.html

[14]S. Lee, T. Kim, K. Kim, and J. Kim, "Lifetime management of flash-based SSDs using recovery-aware dynamic throttling," in FAST, 2012, p. 26.

[15] S.-R. Lee and J.-H. Kim, "Effective Lifetime-Aware Dynamic Throttling for NAND Flash-Based SSDs," 2014.

[16]J. Jeong, S. S. Hahn, S. Lee, and J. Kim, "Lifetime improvement of NAND flash-based storage systems using dynamic program and erase scaling," in FAST, 2014, pp. 61-74.

[17]V. Prabhakaran and T. Wobber, "SSD extension for DiskSim simulation environment," Microsoft Reseach, 2009.

[18] M.-C. Yang, Y.-M. Chang, C.-W. Tsao, P.-C. Huang, Y.-H. Chang, and T.-W. Kuo, "Garbage collection and wear leveling for flash memory: Past and future," in Smart Computing (SMARTCOMP), 2014 International Conference on, 2014, pp. 66-73.

[19]M.-L. Chiang and R.-C. Chang, "Cleaning policies in mobile computers using flash memory," Journal of Systems and Software, vol. 48, pp. 213-231, 1999.

[20]UMass Trace Repository. http://traces.cs.umass.edu/index.php/Storage/Storage

[21]SNIA IOTTA Repository. http://iotta.snia.org/

[22]Intel® High Performance Solid-State Drive Limited Warranties. http://www.intel.com/support/ssdc/hpssd/sb/CS-029645.htm

[23]LIMITED THREE YEAR WARRANTY – CRUCIAL® SSDs. http://www.crucial.com/wcsstore/CrucialSAS/pdf/warranty/crucial-ssd-limited-3-year-warranty-en.pdf