A Set-Based Mapping Strategy for Flash-Memory Reliability Enhancement

隨著快閃記憶體在眾多領域的廣泛應用，可靠性成為了一個關鍵的議題。引發這個研究的動機在於工業界常將具有瑕疵的快閃記憶體「次級化」後售出，但仍需在低成本的考量下保證其產品具有一定的使用壽命。面對此類強烈的需求，我們提出了一套以集合(Set)為主要概念的快閃記憶體管理策略，稱之SFTL，其最大的特色運作時僅需要極少量的資源，如靜態隨機存取記憶體(SRAM)。此外我們也顧及了管理策略的彈性與損耗程度的平均化(Wear-leveling)。我們將所提出的管理策略與目前工業界中許多受歡迎的既存做法相互比較分析，並且以真實的儲存裝置存取記錄(Trace file)實驗，結果顯示SFTL不僅大幅提昇了快閃記憶體的使用壽命，同時也大幅增進了讀取效率。With the wide applicability of flash memory in various application domains, reliability has become a very critical issue. This research is motivated by the needs to resolve the lifetime problem of flash memory and a strong demand in turning thrown-away flash-memory chips into downgraded products. We proposes a set-based mapping strategy with an effective implementation and low resource requirements, such as SRAM. A configurable management design and the wear-leveling issue are considered. The behavior of the proposed method is also analyzed with respect to popular implementations in the industry. We show that the endurance of flash memory can be significantly improved by a series of experiments over a realistic trace. Meanwhile, the read performance is even largely improved in the experiments.1 Introduction ...... 1 2 System Architecture and Motivation ...... 4 2.1 System Architecture ...... 4 2.2 Motivation ...... 6 3 SFTL: An Efficient Set-Based Mapping Strategy ...... 9 3.1 Overview ...... 9 3.2 A Set-Based Mapping Mechanism ...... 11 3.2.1 Logical/Physical Set Tables ...... 11 3.2.2 Physical Block Status: The Un-Index Table ...... 12 3.3 Access Strategy and Mapping Process ...... 14 3.3.1 The Handling of Reads and Writes ...... 14 3.3.2 SearchOffset() and GetFreeBlock() ...... 15 3.4 Implementation Remarks ...... 17 3.4.1 The Memory Space Overhead: UIT ...... 17 3.4.2 Write Latency and Grouping of Physical Blocks ...... 18 4 Performance Evaluation ...... 23 4.1 Experiment Setup ...... 23 4.2 Experiment Result ...... 25 4.2.1 Main-Memory Requirements ...... 25 4.2.2 Flash-Memory Lifetime and Defect-Block Tolerance ...... 28 4.2.3 Access Performance Evaluation ...... 29 4.2.4 NFTL with Set Extensions ...... 31 5 Conclusion ...... 3