Research On Storage Management For Flash Memory

In recent years, flash memory is widely used in various embedded systems and portable devices, due to its non-volatility, high speed, shock resistence, low power consumption, and small size. In early years, because of its small capacity, flash memory is mainly used in cell phones, digital cameras, portable media players, etc. With rapid increase of its density and decrease of its price, flash memory enters into many areas, dominated by magnetic disk in the past few decades.With the emergence of new flash-based applications, flash memory has become an important secondary storage device. Therefore, storage management for flash memory attracts more and more attention from researchers. Due to the differences between flash memory and magnetic disk, traditional storage management techniques, which are based on magnetic disk, could not be directly applied on flash memory. Thus, researches on efficient storage management for flash memory are full of theoretic and practical value.This dissertation summarized the existing researches on storage management for flash memory, and analysed the key problems of storage management, then made deeply study on several key problems, including space alocation, garbage collection and buffer replacement algorithm.Firstly, this dissertation introduced the characteristics of flash memory. There are two types of flash memory, NOR flash and NAND flash. The differences and similarities between them were also introduced. Furthermore, the dissertation introduced the related works on key problems of flash memory storage management.Secondly, space allocation is one of the key problems of storage management. A block-set based storage management was proposed in this dissertation. Logical blocks were classified into hot blocks and cold blocks according to the update loads, and different managements were provided for them. The propose method could achieve a good tradeoff between erase count and space utilization.Thirdly, garbage collection is a hot issue in storage management research. A double-replacement-block based garbage collection mechanism was proposed in this dissertation. Different managements were provided for logical block with different amout of updated pages. The proposed method could significantly decrease valid page copy cost.Finally, buffer replacement algorithm for flash memory was concerned. An expected-flash-access-cost based buffer replacement algorithm was proposed in this dissertation. Evaluating the expected-flash-access-cost of logical pages according to the read/write loads, the proposed algorithm selected the page with least cost to evict. Therefore, the proposed algorithm could significantly decrease flash access cost.The contributions of the dissertation are listed as follows:(1) A block-set based storage management was proposed. Logical blocks were classified into hot blocks and cold blocks according to the update loads, and different managements were provided for them. The propose method could significantly decrease erase count, and keep space utilization at high level.(2) A double-replacement-block based garbage collection was proposed. Different managements were provided for logical block with different amout of updated pages. The proposed method could significantly decrease valid page copy cost, decrease erase count, and increase garbage collection efficiency.(3) An expected-flash-access-cost based buffer replacement algorithm was proposed. Evaluating the expected-flash-access-cost of logical pages according to the read/write loads, the proposed algorithm selected the page with least cost to evict. The proposed algorithm could significantly decrease flash access cost.