Program context-assisted address translation for high-capacity SSDs

As the capacity of NAND flash-based SSDs keeps increasing, it becomes crucial to design a memory-efficient address translation algorithm that offers high performance when a translation table cannot be entirely loaded in a controller DRAM. Existing flash translation layers (FTL) employ demand-based address translation which caches popular mapping information in DRAM by leveraging locality of I/O references. Owing to the lack of information about detailed behaviors of applications, however, existing demand-based FTLs often suffer from many translation-table misses and thus result in sub-optimal performance. In this paper, we propose a new Program context-AssisteD Flash Translation Layer, called PADFTL. Unlike existing FTLs which are implemented as the form of firmware, PADFTL is vertically integrated with a host-level I/O classifier which provides useful hints for an FTL in an SSD to make a better decision in managing a translation table. The host-level I/O classifier monitors unique behaviors of applications by analyzing their program contexts and categorizes I/O patterns into four types, (1) Loop, (2) Hot, (3) Sequential, and (4) Random, which are then delivered to an SSD through extended interfaces. The SSD-side module of PADFTL partitions a controller DRAM into four zones and isolates mapping information associated with different I/O patterns into separate zones. By employing cache management strategies optimized for individual zones, PADFTL can lower the overall translation-table miss ratio. To evaluate the effectiveness of PADFTL, we implement the host-level classifier in the Linux kernel and PADFTL's FTL in a trace-driven FTL simulator. In our experimental results, compared to the state-of-the-art FTL, PADFTL increases the overall table hit ratio by 16% while reducing the address translation time by up to 20% on average.