Browsing by Author "Hahn, Sangwook Shane"

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File Fragmentation in Mobile Devices: Measurement, Evaluation, and Treatment
(2019-09-01)
Ji, Cheng
;
Chang, Li-Pin
;
Hahn, Sangwook Shane
;
Lee, Sungjin
;
Pan, Riwei
Mobile devices, such as smartphones, have become a necessity in our daily life. However, users may notice that after being used for a long time, mobile devices begin to exhibit a sluggish response. Based on an empirical study on a collection of aged smartphones, this work identified that file fragmentation is among the key factors that contribute to the progressive degradation of response time. This study takes a three-step approach: First, this study designed a set of reproducible file-system aging processes based on User-Interface (UI) script replay. Through the aging processes, it confirmed that file fragmentation quickly emerged, and SQLite files were among the most severely fragmented files. Second, based on the workloads of a selection of popular mobile applications, this study observed that file fragmentation did have an impact on user-perceived latencies. Specifically, the launching time of Chrome on an aged file system was 79 percent slower than it was on a pristine file system. Third, this study evaluated existing treatments of file fragmentation, including space preallocation, persistent journal, and file defragmentation to understand their efficacies and limitations. This study also evaluated a state-of-the-art copyless defragmenter, janusd, to show its advantage over the existing methods.
Improving File System Performance of Mobile Storage Systems Using a Decoupled Defragmenter
(2017-01-01)
Hahn, Sangwook Shane
;
Lee, Sungjin
;
Ji, Cheng
;
Chang, Li-Pin
;
Yee, Inhyuk
In this paper, we comprehensively investigate the file fragmentation problem on mobile flash storage. From our evaluation study with real Android smartphones, we observed two interesting points on file fragmentation on flash storage. First, defragmentation on mobile flash storage is essential for high I/O performance on Android smartphones because file fragmentation, which is a recurring problem (even after defragmentation), can significantly degrade I/O performance. Second, file fragmentation affects flash storage quite differently than HDDs. When files are fragmented on flash storage, the logical fragmentation and the physical fragmentation are de coupled and a performance degradation mostly comes from logical fragmentation. Motivated by our observations, we propose a novel defragger, Janus defragger (janusd), which supports two defraggers, janusdL for a logical defragger and janusdP for a physical defragger. JanusdL, which takes advantage of flash storage's internal logical to physical mapping table, supports logical defragmentation without data copies. JanusdL is very effective for most fragmented files while not sacrificing the flash lifetime. JanusdP, which is useful for physically fragmented files but requires data copies, is invoked only when absolutely necessary. By adaptively selecting janusdL and janusd P, janusd achieves the effect of full file defragmentation without reducing the flash lifetime. Our experimental results show that janusd can achieve at least the same level of I/O performance improvement as e4defrag without affecting the flash lifetime, thus making janusd an attractive defragmentation solution for mobile flash storage.