R&D: Seer-SSD – Bridging Semantic Gap Between Log-Structured File Systems and SSDs to Reduce SSD Write Amplification

IEEE Xplore has published, in 2021 IEEE 39th International Conference on Computer Design (ICCD) proceedings, an article written by You Zhou, School of Computer Science and Technology, Huazhong University of Science and Technology, Wuhan, China, Ke Wang, Fei Wu, Changsheng Xie, and Hao Lv, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China.

Abstract: “Log-structured file systems (LS-FSs) sequentialize writes, so they are expected to perform well on flash-based SSDs. However, we observe a semantic gap between the LS- FS and SSD that causes a stale-LBA problem. When data are updated, the LS-FS allocates new logical block addresses (LBAs). The relevant stale LBAs are invalidated and then trimmed or reused with a delay by the LS-FS. During the time interval, stale LBAs are regarded temporarily as valid and migrated unnecessarily by garbage collection in the SSD. Our experimental study of real-world traces reveals that stale-LBA migrations amount to 59%-150% of host data writes. To solve this serious problem, we propose Seer-SSD to deliver stale-LBA metadata along with written data from the LS-FS to the SSD. Then, stale LBAs are invalidated actively and selectively in the SSD without compromising file system consistency. Seer-SSD can be implemented easily based on existing block interfaces and maintain compatibility with non-LS-FSs. We perform a case study on an emulated NVMe SSD hosting F2FS (a state-of-the- art LS-FS). Experimental results with popular databases show that Seer-SSD improves the throughput by 99.8% and reduces the write amplification by 53.6%, on average, compared to a traditional SSD unaware of stale LBAs.“