ZB⁺-tree: A Novel ZNS SSD-Aware Index Structure

ZNS SSD is a newly emerging solid state drive (SSD). It supports zone-based storage and access of data in SSD. Compared with traditional SSDs, ZNS SSD can effectively improve the read-write throughput of SSD, reduce write amplification and over-provisioning in SSD. However, ZNS SSD requires that data must be written to zones sequentially, and tasks such as space allocation and garbage collection need to be controlled by users. These characteristics of ZNS SSD pose new challenges to many components in traditional database systems, such as storage management, indexing, and buffer management. This paper mainly studies how to adapt the traditional B⁺-tree index structure to ZNS SSD. We propose a new ZNS SSD-aware index structure called ZB⁺-tree (ZNS-aware B⁺-tree), which is the first ZNS SSD-aware index so far. ZB⁺-tree takes B⁺-tree as the base structure, and utilizes the two kinds of zones inside ZNS SSD, namely conventional zones supporting a few random writes and sequential zones only supporting sequential writes. In particular, it uses conventional zones to absorb random writes to ZNS SSD and stores the index nodes in conventional and sequential zones separately. We design different node structures for the nodes in the two zones. By using different types of nodes on the two zones, ZB⁺-tree can absorb random writes on the index and ensure the sequential write requirements on sequential zones. We conduct experiments by simulating ZNS SSD using null_blk and libzbd. Also, we modify the existing copy-on-write (CoW) B⁺-tree as the competitor. The results show that ZB⁺-tree outperforms CoW B⁺-tree in various metrics including running time and space efficiency.