Abstract
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.
Highlights
F ILESYSTEM aging, which is called fragmentation, is a phenomenon wherein contents of files are scattered throughout a filesystem instead of being located in one contiguous location
We show how vulnerable log-structured filesystems (LFSs) is to fragmentation, compared with the in-place update filesystem, and measure the performance degradation incurred by filesystem aging with various types of storage devices
anti-aging log-structured filesystem (AALFS): ANTI-AGING LOG-STRUCTURED FILESYSTEM Heretofore, this paper has addressed the problem of filesystem fragmentation from the performance perspective and investigated the rationale behind the performance degradation with various types of storage devices
Summary
F ILESYSTEM aging, which is called fragmentation, is a phenomenon wherein contents of files are scattered throughout a filesystem instead of being located in one contiguous location. We show how vulnerable LFS is to fragmentation, compared with the in-place update filesystem, and measure the performance degradation incurred by filesystem aging with various types of storage devices. Flash-based storage consists of multiple planes and channels which can be accessed in parallel [24]– [26] Utilizing this parallelism, the storage devices pre-fetch subsequent logical pages from the flash chips to DRAM in advance, for the sake of sequential read performance. Optane SSDs usually have a smaller number of parallel units than flash SSDs, and an increase in the number of I/O commands induced by fragmentation can incur resource conflicts inside Optane SSDs. the high performance of Optane SSDs reveals the kernel overhead which is imperceptible on other storage devices
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