Anti-Aging LFS: Self-Defragmentation with Fragmentation-Aware Cleaning

For several decades, filesystem aging has been widely studied, but nonetheless, it still remains an unsolved problem. Among various filesystems, log-structured filesystems have been reported to be vulnerable to fragmentation due to their append-only write policy. Fragmentation hinders various I/O activities such as sequential read and trim operations, regardless of the underlying storage types. This paper extensively analyzes the rationale behind performance degradation incurred by fragmentation on various types of storage devices. To eliminate fragmentation without additional I/O overhead, we propose an anti-aging log-structured filesystem, called AALFS. During segment cleaning, AALFS re-arranges the order of valid blocks based on inode number and file offset to eliminate existing fragmentation. To enhance the efficacy of the re-ordering process, the new victim selection policy of AALFS reflects the fragmentation degree of each segment in the selection of victim segments. Our experimental results show that AALFS effectively eliminates fragmentation by up to 99.8% and significantly improves sequential read performance on various types of storage devices. Particularly, AALFS improves the sequential read throughput of IOzone on hard disk drives by up to 22.8 times.