Fraggle: Reducing File Fragmentation on DRAM-Less SSD

Abstract

Mobile devices adopt DRAM-less solid-state storage drives (SSDs) as the flash storage to reduce power consumption and manufacturing costs, yet DRAM-less SSDs are significantly impacted by file fragmentation, thereby degrading the responsiveness of mobile devices. Existing methods to mitigate file fragmentation, including preallocation and defragmentation, face critical limitations. Preallocation prevents file fragmentation by reserving contiguous free logical block address (LBA) space for individual files. However, in the limited LBA space exposed by the SSD, contiguous free space is gradually consumed and fragmented by scattered data blocks, reducing the effectiveness of preallocation. Defragmentation recovers file access performance by reorganizing fragmented file layouts into contiguous ones. However, when applied to DRAM-less SSDs, defragmentation introduces excessive cache eviction of dirty mapping entries to the slow Nand flash, causing significant performance disruption. This article presents Fraggle, a host-device co-design that mitigates file fragmentation on DRAM-less SSDs by preserving the efficacy of preallocation within an extended LBA space. To effectively manage the extended LBA space, Fraggle introduces two key components: 1) HashFTL, a compact, cache-efficient flash translation layer (FTL) mapping mechanism specifically optimized for DRAM-less SSDs and 2) a device-assisted space allocator that simultaneously reduces space allocation overhead and resolves layout imbalance issues in HashFTL caused by sparse LBA distribution. Evaluation results demonstrate that, compared to F2FS and state-of-the-art preallocation methods, Fraggle accelerates execution time by up to $9.26\times $ and $8.07\times $ under SQLite and real-world mobile application workloads.