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SHRD: Improving Spatial Locality in Flash Storage Accesses by Sequentializing in Host and Randomizing in Device

Recent advances in flash memory technology have reduced the cost-per-bit of flash storage devices such as solid-state drives (SSDs), thereby enabling the development of large-capacity SSDs for enterprise-scale storage. However, two major concerns arise in designing SSDs. The first concern is the poor performance of random writes in an SSD. Server workloads such as databases generate many random writes; therefore, this problem must be resolved to enable the usage of SSDs in enterprise systems. The second concern is that the size of the internal DRAM of an SSD is proportional to the capacity of the SSD. The peculiarities of flash memory require an address translation layer called flash translation layer (FTL) to be implemented within an SSD. The FTL must maintain the address mapping table in the internal DRAM. Although the previously proposed demand map loading technique can reduce the required DRAM size, the technique aggravates the poor random performance. We propose a novel address reshaping technique called sequentializing in host and randomizing in device (SHRD), which transforms random write requests into sequential write requests in the block device driver by assigning the address space of the reserved log area in the SSD. Unlike previous approaches, SHRD can restore the sequentially written data to the original location without requiring explicit copy operations by utilizing the address mapping scheme of the FTL. We implement SHRD in a real SSD device and demonstrate the improved performance resulting from SHRD for various workloads.