ZNS+: Advanced zoned namespace interface for supporting in-storage zone compaction

The NVMe zoned namespace (ZNS) is emerging as a new storage interface, where the logical address space is divided into fixed-sized zones, and each zone must be written sequentially for flash-memory-friendly access. Owing to the sequential write-only zone scheme of the ZNS, the log-structured file system (LFS) is required to access ZNS solid-state drives (SSDs). Although SSDs can be simplified under the current ZNS interface, its counterpart LFS must bear segment compaction overhead. To resolve the problem, we propose a new LFS-aware ZNS interface, called ZNS+, and its implementation, where the host can offload data copy operations to the SSD to accelerate segment compaction. The ZNS+ also allows each zone to be overwritten with sparse sequential write requests, which enables the LFS to use threaded logging-based block reclamation instead of segment compaction. We also propose two file system techniques for ZNS+-aware LFS. The copyback-aware block allocation considers different copy costs at different copy paths within the SSD. The hybrid segment recycling chooses a proper block reclaiming policy between segment compaction and threaded logging based on their costs. We implemented the ZNS+ SSD at an SSD emulator and a real SSD. The file system performance of the proposed ZNS+ storage system was 1.33–2.91 times better than that of the normal ZNS-based storage system.