A Buffer Management Algorithm Based On NAND Flash

The SSD (Solid Storage Disk) based on NAND flash has shown many advantagesover traditional magnetic hard disk, such as better random read performance, low powerconsumption, good shock resistance and so on. However, SSD suffers performancedegradation under random write requests, due to the intrinsic weak points of itself, suchas erase in blocks, read/write in pages, asymmetric read/write/erase cost, and limitederase cycles. The problem is serious, especially when multi-channel architecture isadopted. For these reasons, buffer is took into SSD.In this paper, some block-level buffer management algorithms (for instance, FABand CLC) and Page Padding policy are analyzed. However, these existing algorithmsstill have room for improvement. A new buffer management algorithm is designedbased on the improvement of them, which takes the features of NAND flash and themulti-channel architecture into account.(1) In order to effectively execute eviction, A preserve-priority-shift bufferreplacement algorithm is proposed based on CLC. A threshold, based on recencyinstead of the length of list, is set to decide the priority of page clusters in buffer. Theentities are divided into two different states, managed by two lists, and evicted byappropriate principle. The preserve priority of page clusters can be changed accordingto access requires. It can enhance hit ratio and reduce the number of erase under thecondition of random small-size data.(2) Double-threshold Page Padding policy is proposed in this paper by improvingsingle-threshold Page Padding policy. To execute full Page Padding or partial PagePadding is judged by the two thresholds. In order to get the appropriate expression, thetime-cost of different Page Padding policies in buffer management and garbagecollection are analyzed, and the value of thresholds are given by simulation. Under thecondition of random small-size data, double-threshold page padding policy is useful toreduce the number of erase.Finally, the buffer management algorithm including preserve-priority-shift bufferreplacement algorithm and double-threshold policy is evaluated in a test SSD, in whichmulti-channel architecture is adopted. The results show that the buffer managementalgorithm proposed in this paper can effectively improve performance of SSD.