Slack Space Recycling: Delaying On-Demand Cleaning in LFS for Performance and Endurance

Yongseok OH  Jongmoo CHOI  Donghee LEE  Sam H. NOH  

Publication
IEICE TRANSACTIONS on Information and Systems   Vol.E96-D   No.9   pp.2075-2086
Publication Date: 2013/09/01
Online ISSN: 1745-1361
DOI: 10.1587/transinf.E96.D.2075
Print ISSN: 0916-8532
Type of Manuscript: PAPER
Category: Data Engineering, Web Information Systems
Keyword: 
Log-structured File System,  LFS cleaning,  garbage collection,  endurance,  slack space recycling,  solid state drive,  

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Summary: 
The Log-structured File System (LFS) transforms random writes to a huge sequential one to provide superior write performance on storage devices. However, LFS inherently suffers from overhead incurred by cleaning segments. Specifically, when file system utilization is high and the system is busy, write performance of LFS degenerates significantly due to high cleaning cost. Also, in the newer flash memory based SSD storage devices, cleaning leads to reduced SSD lifetime as it incurs more writes. In this paper, we propose an enhancement to the original LFS to alleviate the performance degeneration due to cleaning when the system is busy. The new scheme, which we call Slack Space Recycling (SSR), allows LFS to delay on-demand cleaning during busy hours such that cleaning may be done when the load is much lighter. Specifically, it writes modified data directly to invalid areas (slack space) of used segments instead of cleaning on-demand, pushing back cleaning for later. SSR also has the added benefit of increasing the lifetime of the now popular SSD storage devices. We implement the new SSR-LFS file system in Linux and perform a large set of experiments. The results of these experiments show that the SSR scheme significantly improves performance of LFS for a wide range of storage utilization settings and that the lifetime of SSDs is extended considerably.