Seer-SSD: Bridging Semantic Gap between Log-Structured File Systems and SSDs to Reduce SSD Write Amplification

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.