Optimizing CoW-based File Systems on Open-Channel SSDs with Persistent Memory

Block-based file systems, such as Btrfs, utilize the copy-on-write (CoW) mechanism to guarantee data consistency on solid-state drives (SSDs). Open-channel SSD provides opportunities for in-depth optimization of block-based file systems. However, existing systems fail to co-design the two-layer semantics and cannot take full advantage of the open-channel characteristics. Specifically, synchronizing an overwrite in Btrfs will copy-on-write all pages in the update path and induce severe write amplification. In this paper, we propose a hybrid fine-grained copy-on-write and journaling mechanism (HyFiM) to address these problems. We first utilize persistent memories to preserve the address mapping table of open-channel SSD. Then, we design an intra-FTL copy-on-write mechanism (IFCoW) that eliminates the recursive updates caused by overwrites. Finally, we devise fine-grained metadata journals (FGMJ) to guarantee the consistency of metadata with minimum overhead. We prototype HyFiM based on Btrfs in the Linux kernel. Comprehensive evaluations demonstrate that HyFiM can outperform over Btrfs by 30.77% and 33.82% for sequential and random overwrites, respectively.