The Researh On Reliability Enhancement Of MLC NAND Flash Storage System

As a semiconductor-based nonvolatile memory, NAND flash memory features non-volatility, low power-consumption, fast access time and shock-resistance. Therefore NAND flash memory has been widely used as the storage media in storage field especially test data storage field. MLC(Multi Level Cell) NAND flash devices can cost less and have higher storage density than SLC(Single Level Cell) NAND flash devices. Therefore, MLC flash devices have been widely adopted by the market, and have become the main stream NAND flash storage media. The dramatic improvements in the price and capacity of MLC NAND flash memory have been achieved by technological advances, such as the aggressive shrinking of process geometry and an increase in the number of bits stored in each memory cell. These technological advances inevitably degrade the reliability of MLC NAND flash storage system.MLC NAND flash normally has three reliability issues: the rising BER(Bit Error Rate), the decrease of the lifetime and the data integrity issue during power loss. This thesis firstly systematically analyzes mechanisms of these reliability issues and conducts survey for the recent work. With awareness of these related work, this thesis proposes approaches to cope with these reliability issues for MLC NAND flash storage systems.We firstly conduct research to reduce the BER of MLC NAND flash storage system. While the traditional redundancy methods will induce great storage space overhead for the system, we propose a metadata aware strategy and only backup critical data to reduce the storage space overhead. The traditional redundancy methods also will sharply degrade the I/O performance of the storage system. We observe that the error distribution of MSB pages and LSB pages is asymmetric. This thesis utilizes this feature and allocates critical data to physical pages with lower BER to reduce the uncorrectable pages of the system and meanwhile induce limited I/O performance for the storage system. We also designed a hardware platform to evaluate all of our schemes or methods.Then we present an approach to prolong the lifetime of MLC NAND flash storage system. This thesis presents Garbage Collection Aware Stripping(GCAS) method, a novel stripping method that considers the efficiency of garbage collection for NAND flash memory based SSD. GCAS exploits the feature of the out-of-order processing of the request queue and reschedules the requests to achieve the same garbage collection efficiency as non-stripping method, while obtaining better I/O performance than the current stripping methods. GCAS reduces the overhead of garbage collection process and ultimately prolongs the lifetime of MLC NAND flash storage system.We also propose a novel method called Geedy Backup to ensure data integrity during power loss. In Geedy Backup, by exploiting the feature of the discharge process of capacitors, all available SSD channels are fully utilized and dynamically adjusted based on the available current, data can be efficiently stored in a reliable manner. Moreover, in Geedy Backup, to write data in a time/energy-efficient manner with higher reliability and meanwhile eliminate retroactive data corruption, we adopt a fast and reliable programming strategy. To the best of our acknowledge, this is the first work to jointly utilize the property of the multi-page architecture of MLC NAND flash and the discharge characteristic of capacitors to improve the efficiency and reliability of the backup process in SSDs with backup capacitors.