Tiered Hashing: Revamping Hash Indexing under a Unified Memory-Storage Hierarchy

NAND flash-based Solid State Drives (SSDs) provide a promising opportunity to enable the unified memory-storage hierarchy (UMH). The UMH renders a single memory address space for heterogeneous memories. Thus, the CPUs can directly access structured data in SSDs and eliminate bulk data copy/swap between the memory and storage devices. However, applying traditional indexing structures directly on SSDs may lead to poor performance. Particularly, the popular hash indexing generates highly randomized write traffic, incurring significant garbage collection overhead in SSDs. To address this problem, we propose a novel SSD-friendly hash indexing scheme called Tiered Hashing. It employs a multi-layer structure and opportunistic data movement (ODM) to construct skewed writes. Hence, the SSD can transform the writes into multi-streamed writes, where hot and cold data are separated to reduce GC overhead. Experimental results show Tiered Hashing reduces the average write latency and GC overhead by up to 94.98% and 90.71% compared to state-of-the-art hash indexings, without sacrificing read performance.