安全刪除的效率與速度取決於基礎底層儲存裝是的資料擺放方式。由於新興的疊瓦磁記錄（Shingled Magnetic Recording, SMR）技術的順序寫入限制，不適當的資料擺放方式可能導致嚴重的寫入放大 (write amplification) 問題並減低安全刪除的性能。由於檔案系統元資料的小容量性質，當我們需要安全擦除已刪除文件之檔案系統之元資料時，會進一步使得疊瓦式硬碟上執行安全刪除的性能急劇的下降。這樣觀察促使我們提出一種新的空間分配（SMR-aware space allocation, SSSA）策略，以促進安全刪除已刪除文件及其元資料之過程。我們所提出的策略被加入到廣泛使用的擴展文件系統4（Extended File System 4, ext4）中，並通過一系列實驗進行了評估以證明所提出策略的有效性。實驗結果顯示，與單純的基於疊瓦式硬碟的ext4檔案系統相比，我們所提出的SSSA策略可平均降低91.3% 的安全刪除時間。

The efficiency of secure deletion is highly dependent on the data layout of underlying storage devices. In particular, owing to the sequential-write constraint of the emerging Shingled Magnetic Recording (SMR) technology, an improper data layout could lead to serious write amplification and hinder the performance of secure deletion. The performance degradation of secure deletion on SMR drives is further aggravated with the need to securely erase the file system metadata of deleted files due to the small-size nature of file system metadata. Such an observation motivates us to propose a secure-deletion and SMR-aware space allocation (SSSA) strategy to facilitate the process of securely erasing both the deleted files and their metadata simultaneously. The proposed strategy is integrated within the widely-used extended file system 4 (ext4) and is evaluated through a series of experiments to demonstrate the effectiveness of the proposed strategy. The evaluation results show that the proposed strategy can reduce the secure deletion latency by 91.3% on average when compared with naive SMR-based ext4 file system.

Table of Contents
1. Introduction----------------------------------------------3
2. Background and Motivation---------------------------------7
2.1 Shingled Magnetic Recording Drives--------------------7
2.2 Extended File System 4--------------------------------8
2.3 Motivation--------------------------------------------10
3. Secure-Deletion and SMR-Aware Space Allocation Strategy---12
3.1 Overview----------------------------------------------12
3.2 Metadata Redirection Mechanism------------------------14
3.3 Elastic Guard Barrier Scheme--------------------------15
3.4 Anti-Fragmentation Space Allocator--------------------16
4. Performance Evaluation------------------------------------19
4.1 Experimental Setup------------------------------------19
4.2 Experimental Results----------------------------------20
5. Conclusion------------------------------------------------24
6. Acknowledgement-------------------------------------------25
7. References------------------------------------------------26

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