Design and Implementation of FTL-based Flash Memory Management with Fast Cleaning

[[abstract]] 由於非揮發性、重量輕、耐震、及低耗能等特性，快閃記憶體廣泛地成為嵌入式系統和行動裝置之儲存媒體。然而快閃記憶本身無法像磁碟一樣原地更新資料，而且每個區塊具有抹除次數之限制。所以要讓快閃記憶體能有效率的運作，快閃記憶體需要一個不同儲存空間管理方式且針對其特性所設計。許多研究採用log-based的儲存空間管理方式，因此需要一個有效率的清除策略來回收那些己經陳舊無效的資料所佔據之空間。 本論文中，我們設計和實作了一個新的快閃記憶體轉換層來管理快閃記憶體的儲存空間，並提出了一個快速清除機制。我們利用DAC的方法將快閃記憶體中的資料依群組做冷熱門區分。對於具有大容量的NAND型快閃記憶體，我們的快閃記憶體儲存管理系統使用了多重LRU鏈結串列的方式，以減少在進行空間回收時，選擇清除區塊的時間。此外，針對區塊清除策略，我們修改了cost-benefit策略，並配合區塊冷熱門程度給予不同的權重值來選擇回收區塊。實驗結果顯示我們所提出的快閃記憶體儲管理系統以及快速清除機制可以有效的減少快閃記憶體的抹除次數、加快區塊清除速度，並提升快閃記憶體儲存系統的效率。[[abstract]] Because of the advantages of non-volatility, lightweight, low power consumption, and shock resistance, flash memory has been widely used as the storage of embedded systems and mobile devices. However, unlike hard disk, flash memory does not support update-in-place, and each block on flash memory has the limitation of erasure times. Therefore, flash memory needs a different storage management scheme designed specifically for the sake of flash memory characteristics. Many researches adopt the log-based approach which needs an efficient cleaning policy to reclaim the storage space occupied by obsolete data. In this thesis, we have designed and implemented a new flash translation layer for flash memory management and proposed a fast cleaning mechanism for space reclamation. It is based on the DAC technique to separately cluster hot and cold data in flash memory. For the NAND-type flash memory with large capacity, it maintains multiple LRU lists to reduce the time of selecting blocks for erasure in cleaning activity. Moreover, we modify the cost-benefit policy with different weight that considers the attribute of each block on flash memory in selecting a block for cleaning. Simulation results show that our proposed flash memory management scheme with fast cleaning mechanism could efficiently decrease the number of erase operations, speed up the cleaning activity, and enhance system performance.[[note]]碩