ROWE-tree: A Read-Optimized and Write-Efficient B+-tree for Persistent Memory

Persistent memory (PM) exhibits a huge potential to provide B+-tree indexes with high performance, efficient persistence, and instant recovery. A large number of PM-optimized B+-tree indexes have been proposed, but most of them fail to provide high performance for both read and write operations because: (1) their designs of search optimization and insert improvement are often traded off against each other, and (2) they overlook the read/write interference problem of PM which incurs unpredictable performance degradation. In this paper, we propose ROWE-tree, a read-optimized and write-efficient B+-tree for PM. The designs of our ROWE-tree consist of three key points. First, we propose two techniques to make a good trade-off between write and read performance: self-verifying insertion, which reduces consistency overhead by using the key itself as a persist mark instead of additional metadata, and semi-sorted leaf nodes, which use append-only insertion to avoid the shifting overhead of sorting nodes but keep intra-cache-line items sorted to accelerate the lookup. Second, based on the observation that data accesses are highly skewed in real-world workloads, we build an in-DRAM cache of hot items to outsource accesses to hot items to DRAM. By doing so, we can alleviate the read/write interference of PM and significantly improve overall performance. Third, to cope with the dynamic changes of hot items, we exploit a lightweight mechanism to track such changes at run-time. Using Intel Optane DCPMM, our evaluations show that ROWE-tree obtains up to 3.86 × higher performance than the state-of-the-art PM B+-tree indexes under YCSB workloads.