Study On Mapping Strategy For FTL Of SSD

Solid state hard drive has been widely used in military, automotive, videosurveillance, network monitoring, network terminals, poweraviation and other fieldsbecause of its fast read and write speeds, low power consumption, no noise,anti-vibration, low-calorie, small size and non-volatile characteristics. The file systemused in HDD cannot be directly applied into SSD for the features of erasing beforewriting and a limited number of erasion of NAND FLASH and the physical structureof SSD. The program to solve these problems is to add Flash Translation Layer(FTL)above the flash memory device that is modeled to block device structure as HDD.Sothe traditional storage management technology can be applied directly to the flashmemory device.FTL plays an important role in NAND flash management.In recent years, threetypes of FTL mapping algorithms have been proposed that are the page mapping, theblock mapping and the hybrid mapping. The page mapping can map any logical pageto any physical page which is efficient and flexible. But it will consume a lot ofexpensive SRAM. The logical blocks and physical blocks can be mapped and eachpage can only be mapped to a physical block in a fixed position for the block mapping,which saves a lot of SRAM but to reduce the many pages of mapping opportunities. Itwill lead to frequent garbage collection and affect the system performance. In thehybrid mapping a small part of the recently updated pages are the page mapping. Therest of the pages are used the block mapping. But when garbage collection, the logic iscomplex. It requires a variety of merge operations and lead to performance degradation.The mainstream approach is that the recently used mapping tables as hot data arecached in the limited SRAM and then most of the mapping tables are put in the flashmemory. It depends on locality to speed up the mapping and increase the IOPS.The main contents of this thesis include four aspects:①SAS interface technology and multi-channel parallel technology were givenintensive study, which were the key to performance improvement. The principle ofgarbage collection and the strategy of wear leveling for solid-state storage and relatednotes were researched.②The need of the flash translation layer for SSD and its basic principle wereresearched. And the common and mainstream FTL algorithms and related technical problems and solutions were studied deeply. There were implements of the correlationalgorithms on the Flashsim in this thesis.③A new mapping strategy for FTL was proposed. And there were the study ofthe DFTL algorithm and the limitations of DFTL in this thesis. The improved ADFTLalgorithm was taken into account time and spatial locality of data access. Comparewith the strategy of DFTL, ADFTL improved effectively the Cache hit rate, reducedthe GC load, improved system response time and reduced the numbers of erase block.The simulation results under Flashsim proved the effectiveness of the mappingstrategies for flash translation layer.