ListDB: Union of Write-Ahead Logs and Persistent SkipLists for Incremental Checkpointing on Persistent Memory

Due to the latency difference between DRAM and nonvolatile main memory (NVMM) and the limited capacity of DRAM, incoming writes are often stalled in LSM treebased key-value stores. This paper presents ListDB, a writeoptimized key-value store for NVMM to overcome the gap between DRAM and NVMM write latencies and thereby, resolve the write stall problem. The contribution of ListDB consists of three novel techniques: (i) byte-addressable Index-Unified Logging, which incrementally converts write-ahead logs into SkipLists, (ii) Braided SkipList, a simple NUMAaware SkipList that effectively reduces the NUMA effects of NVMM, and (iii) Zipper Compaction, which moves down the LSM-tree levels without copying key-value objects, but by merging SkipLists in place without blocking concurrent reads. Using the three techniques, ListDB makes background compaction fast enough to resolve the infamous write stall problem and shows 1.6x and 25x higher write throughputs than PACTree and Intel Pmem-RocksDB, respectively.