UHS: An Ultra-fast Hybrid Storage Consolidating NVM and SSD in Parallel

Non-Volatile Memory (NVM) with persistency and near-DRAM performance has been commonly used as first-level fast storage atop Solid-State Drives (SSDs) and Hard Disk Drives (HDDs), constituting classic hierarchy architecture to achieve high cost-performance. However, such NVM/SSD tiered storage overuses primary NVM with limited actual performance and under-utilizes secondary SSD with increasing bandwidth. Besides, NVM and SSD exhibit distinguished I/O characteristics, but are complementary for different I/O patterns. This motivates us to design a superior hybrid storage to fully exploit NVM and SSD simultaneously. In this paper, we propose UHS, an Ultra-fast Hybrid Storage consolidating NVM and SSD to reap their own merits with key enabled techniques. First, UHS builds a uniform yet heterogenous block-level storage view for the upper applications, e.g., file systems or key-value stores. UHS provides static address-mapping to explicitly partition the global block-space into coarse-grain NVM-zones and SSD-zones, which mainly serve the metadata and file data respectively. Second, UHS presents a fine-grain request-level NVM buffer to dynamically absorb small file-writes in runtime and then migrates them to the SSDs in the background. Third, UHS designs I/O-affinity write allocation and hash-based buffer indexing to trade off write gain and read cost of the NVM-buffer. Finally, UHS designs a multi-thread I/O model to take full advantage of parallelism in both NVM and SSD. We implement UHS and evaluate it under a variety of workloads. The experiments show that UHS outperforms SSD, NVM, Bcache-writeback (representative hierarchy storage), and Device-Mapper (state-of-the-art hybrid storage) up to 8X, 1.5X, 3.5X, and 6X respectively.