The emergence of non-volatile memory (NVM) has introduced new opportunities for performance optimizations in existing storage systems. To better utilize its byte-addressability and near-DRAM performance, NVM can be attached on the memory bus and accessed via load/store memory instructions rather than the conventional block interface. In this scenario, a cache line (usually 64 bytes) becomes the data transfer unit between volatile and non-volatile devices. However, the failure-atomicity of write on NVM is the memory bit width (usually 8 bytes). This mismatch between the data transfer unit and the atomicity unit may introduce write amplification and compromise data consistency of node-based data structures such as B+-trees. In this paper, we propose WOBTree, a Write-Optimized B+-Tree for NVM to address the mismatch problem without expensive logging. WOBTree minimizes the update granularity from a tree node to a much smaller subnode and carefully arranges the write operations in it to ensure crash consistency and reduce write amplification. Experimental results show that compared with previous persistent B+-tree solutions, WOBTree reduces the write amplification by up to 86× and improves write performance by up to 61× while maintaining similar search performance.

非易失性存储器 (NVM) 的出现为现有存储系统的性能优化带来了新的机会。为了更好地利用其字节寻址能力和接近 DRAM 的性能，NVM 可以连接到内存总线上，并通过加载/存储内存指令而不是传统的块接口进行访问。在这种情况下，缓存行（通常为 64 字节）成为易失性和非易失性设备之间的数据传输单元。但是，在 NVM 上写入的失败原子性是内存位宽（通常为 8 个字节）。数据传输单元和原子性单元之间的这种不匹配可能会引入写入放大并危及基于节点的数据结构（例如 B+ 树）的数据一致性。在本文中，我们提出了 WOBTree，一种用于 NVM 的写优化 B+-树，以解决不匹配问题而无需昂贵的日志记录。WOBTree 最小化了从树节点到更小的子节点的更新粒度，并仔细安排其中的写入操作，以确保崩溃一致性并减少写入放大。实验结果表明，与之前的持久化B+树解决方案相比，WOBTree在保持相似搜索性能的同时，将写放大降低了86倍，将写性能提高了61倍。