Hi-ZNS: High Space Efficiency and Zero-Copy LSM-tree-based Stores on ZNS SSDs

The Zoned Namespace (ZNS) SSD is a newly introduced storage device and provides several new ZNS commands to upper-level applications. Zone-reset command is one of the ZNS commands to erase all the flash blocks within a zone. Since data is grouped and erased in zone units, ZNS SSDs are widely used in LSM-tree-based stores. However, the basic invalidated unit in LSM-tree is an SST/WAL file, which mismatches the erasing unit of a ZNS SSD. Placing different SST/WAL files in the same zone, LSM-tree on ZNS SSDs faces dramatic space amplification and extensive data migration problems. To solve these problems, Hi-ZNS aligns zone sizes with varying-sizes SST/WAL files without changing the ZNS specification. The basic idea of Hi-ZNS is to put an SST/WAL file in a single zone. When an SST/WAL is invalidated, upper-level applications with HiZNS send the zone-reset command to a ZNS SSD device mindlessly without any space amplification and data migration. Specifically, Hi-ZNS allocates physical resources for the zone on demand and provides an infinite logical zone number to avoid wasting storage resources. The extensive evaluation demonstrates that Hi-ZNS substantially improves space efficiency and completely eliminates data migration. Compared with the existing ZenFS&RocksDB, HiZNS increases the maximum completed requests by up to 2.97x and improves the I/O performance by up to 7%.