Mitigating Synchronous I/O Overhead in File Systems on Open-Channel SSDs

Synchronous I/O has long been a design challenge in file systems. Although open-channel solid state drives (SSDs) provide better performance and endurance to file systems, they still suffer from synchronous I/Os due to the amplified writes and worse hot/cold data grouping. The reason lies in the controversy design choices between flash write and read/erase operations. While fine-grained logging improves performance and endurance in writes, it hurts indexing and data grouping efficiency in read and erase operations. In this article, we propose a flash-friendly data layout by introducing a built-in persistent staging layer to provide balanced read, write, and garbage collection performance. Based on this, we design a new flash file system (FS) named StageFS, which decouples the content and structure updates. Content updates are logically logged to the staging layer in a persistence-efficient way, which achieves better write performance and lower write amplification. The updated contents are reorganized into the normal data area for structure updates, with improved hot/cold grouping and in a page-level indexing way, which is more friendly to read and garbage collection operations. Evaluation results show that, compared to recent flash-friendly file system (F2FS), StageFS effectively improves performance by up to 211.4% and achieves low garbage collection overhead for workloads with frequent synchronization.