High performance storage system based on nand flash memory

Abstract

Many mobile devices demand a large-capacity and high-performance storage system in order to store, retrieve, and process large multimedia data quickly. In those devices, NAND flash memory is already becoming one of the most common storage medium because of its versatile features such as non-volatility, solid-state reliability, low power consumption, shock resistance, and high cell densities. NAND flash memory, however, has a restriction that a page, which is the basic unit of read and write operations, should be erased before being rewritten in the same location. This characteristic is sometimes called erase-before-write. Moreover, the erase operations can only be performed on a larger block than the page. Therefore, an intermediate software layer called a flash translation layer (FTL) is usually employed to hide the limitation of erase-before-write. In this thesis, we present a high-performance NAND flash-based storage system based on a multi-channel architecture and proposes a novel superblock-based FTL scheme, which combines a set of adjacent logical blocks into a superblock. The proposed system consists of multiple independent channels, where each channel has multiple NAND flash memory chips. On this hardware, we investigate three optimization techniques to exploit I/O parallelism: striping, interleaving, and pipelining. By combining all the optimization techniques carefully, our system has shown 3.6 times higher overall performance compared to the conventional single-channel architecture. In the proposed FTL scheme, superblocks are mapped at coarse granularity, while pages inside the superblock are mapped freely at fine granularity to any location in several physical blocks. To reduce extra storage and flash memory operations, the fine-grain mapping information is stored in the spare area of NAND flash memory. This hybrid mapping technique has the flexibility provided by fine-grain address translation, while reducing the memory overhead to the leve...