Self-healing SSD Optimization Based On Dwell Time Differentiation

As the cost of flash memory chips drops,flash-based solid state drives(SSD)have become more and more widely used in storage systems.In recent years,with the improvement of flash memory chip manufacturing processes,the density of flash memory and the storage capacity of SSD have been further improved,but the limit on the number of Program/Erase(P/E)cycles of SSD have also been correspondingly reduced.However,there is a self-healing effect in SSD: during the idle time(dwell time)between consecutive P/E cycles,the damage of the dielectric in the flash memory unit can be partially repaired.Since high temperature can improve the self-healing effect more efficiently,self-healing SSD were invented.However,the self-healing SSD perform heating operations on all worn flash memory blocks without considering the data retention requirements,and measure the lifetime of the flash memory based on the worst-case self-healing effect,resulting in many unnecessary heating operations.Considering the impact of the above unnecessary heating operations on the performance of self-healing SSD,it is still an important challenge to study how to maximize the advantages of self-healing SSD and avoid their inherent defects in modern storage systems.In order to build a high-performance self-healing SSD storage system,this thesis proposes a heating scheme for dwell time differentiation,using dwell time variation and flash memory Process Variation(PV)characteristics to improve I/O performance and the lifetime of self-healing SSD.First,it tracks the dwell time of all worn flash memory blocks,predicts their reliability,determines the worn flash memory blocks that need to be heated based on the reliability,and avoids performing heating operations on the worn flash memory blocks that still have high reliability.Then,by analyzing the PV characteristics of the flash memory block,the data of the hot and cold write request is allocated to the flash memory block with matching PV characteristics,further reducing the number of heating.The final experimental results show that the heating scheme for dwell time differentiation proposed in this thesis reduces the number of heating operations by 10.3% on average,boosts the read and write performance by 19.8% and10.2% respectively,and improves the lifetime of flash memory by 17.2% compared with the conventional heating scheme.