LBZ: A Lightweight Block Device for Supporting F2FS on ZNS SSD

Zoned NameSpace SSD (ZNS) is emerging as a new paradigm for serving log-structured storage system (e.g. F2FS). However, existing ZNS SSD cannot directly support random write, which is still highly required by the metadata region of log-structured storage system. In this paper, we propose LBZ, a lightweight in-kernel block device running on ZND SSD with the log-structured design to allow random writes. Specifically, we design LBZ to hold the metadata area of F2FS such that F2FS can run on single ZNS SSD without using extra conventional SSD or conventional namespace of ZND SSD. LBZ first utilizes the out-of-band area of NAND flash chips to maintain the mappings between logical blocks and physical zone offsets. Since logical blocks of LBZ are sequentially written into ZNS SSD, LBZ incurs garbage collection overhead when logical blocks become invalid. In order to support efficient garbage collection in LBZ, we then propose multi-stream separation to distinguish different metadata areas of F2FS and place metadata with similar lifetimes into same zones. Meanwhile, LBZ executes three-phase checkpoint to get aware of invalid blocks and reclaim the blocks itself without modifying F2FS. We implement LBZ on real ZNS SSD and evaluate its basic performance as well as its garbage collection efficiency. The experiment results show LBZ can fully exploit the bandwidth of ZNS SSD. Moreover, LBZ provides efficient garbage collection by reducing the number of written blocks by up to 19x.