非揮發性記憶體(Non-volatile random-access memory/NVRAM)變為嵌入式系統的主流儲存裝置，由於其優越的特性，例如：尺寸小、低功耗、低讀取寫入延遲。和動態隨機存取記憶體(DRAM)不同，大部分的NVRAM都有非對稱性的效能及能量消耗。通常在NVRAM上，一個寫入的操作與讀取的操作相比會消耗更多的能量與時間。不幸的是現在嵌入式與行動裝置的檔案系統，像是EXT2/3和EXT4都不利於NVRAM，原因是現在的行動與嵌入式裝置使用了日誌型檔案系統，雖然日誌型檔案系統提高了檔案系統的資料安全性，但在寫入日誌的兩階段：commit和checkpoint，會造成相同資料的二次寫入，增加額外的寫入。雖然有許多減少寫入操作的相關研究被提出，這些仍然不能有效地降低日誌型檔案系統造成的寫入放大。這樣的觀察促使了這篇論文提出兩階段的縮減寫入量的日誌型檔案系統，叫做wrJFS。在第一個階段，wrJFS將資料區分成兩類：元資料(Metadata)和用戶資料(Userdata)。因為元資料的大小通常非常小（少量的位元組），故使用位元組日誌(Byte-enable journaling)機制來處理元資料。相對的用戶資料是遠大於元資料，將於在第二階段時被硬體編碼器壓縮，這樣一來可以避免寫入放大。wrJFS在一個EXT3的模擬器進行並運行了一系列的實驗。實驗結果顯示，wrJFS與EXT3比較，可以平均89.7％減少寫入操作。

Non-volatile random-access memory (NVRAM) becomes a mainstream storage device in embedded systems due to its favorable features, such as small size, low power consumption and short read/write latency. Unlike dynamic random access memory (DRAM), the most NVRAM has asymmetric performance and energy consumption on read/write operations. Generally, on NVRAM, a write operation consumes more energy and time than a read operation. Unfortunately, current mobile/embedded file systems, such as EXT2/3 and EXT4, are very unfriendly for NVRAM devices. The reason is that in order to increase the reliability of file systems, current mobile/embedded file systems employ a journaling mechanism. Although a journaling mechanism raises the safety of data in a file system, it also writes the same data twice during data commitment and checkpoint. Though several related works have been proposed to reduce the size of a write operation, they still cannot effectively minimize the write amplification of a journaling mechanism. Such observations motivate this paper to design a 2-phase write-reduction journaling file system called wrJFS. In the first phase, wrJFS classified data into two categories: metadata and user data. Because the size of metadata is usually very small (few bytes), the metadata will be handled by partial byte-enabled journaling strategy. In contrast, the size of user data is very large relative to metadata; thus, user data will be processed in the second phase. In the second phase, user data will be compressed by hardware encoder so as to reduce the write size, and managed compressed-enabled journaling strategy to avoid the write amplification. The proposed wrJFS is conducted on an EXT3 simulator and runs a series of experiments. The experimental results show that the proposed wrJFS can reduce the size of the write request by 89.7% on average, compared with the original EXT3.

摘 要 i
Abstract ii
Acknowledgements iv
Table of Content v
List of Figures vii
List of Tables viii
Chapter 1. Introduction 1
Chapter 2. Background and Motivation 6
2.1 NVRAM Characteristics 6
2.2 Journaling file system 8
2.3 Research Motivation 11
Chapter 3. wrJFS: Write-Reduction Strategy 14
3.1 Overview 14
3.2 Design of wrTransaction 18
3.3 Write-reduction Journaling Manger 20
3.3.1 Byte-enabled Journaling Strategy 21
3.3.2 Compressed-enabled Journaling Strategy 23
3.4 Space Manager for Sub-blocks 26
3.5 Journaling Operations with wrJFS 29
3.6 Remark of Uncompressed Data 32
Chapter 4. Performance Evaluation 35
4.1 Experiment Setup 35
4.2 Experimental Results on Write Reduction 38
Chapter 5. Conclusion 45
References 46

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