Research On Low Wear Data Coding For Flash Memory

Flash memory-based solid state disk(NAND Flash-based Solid State Disk / Drive)is widely used in portable devices,personal computers and enterprise-level storage systems due to its low latency,low energy consumption,high shock resistance,etc.,and gradually replaces traditional magnetic hard drives,occupies an important position in the storage market.The solid-state disk controller changes the voltage of the flash memory cells to store data through paired program/erase operation,and each pair of program/erase operation causes cumulative wear on the flash memory cells.However,the wear that the flash memory cell can withstand is limited.Once the wear of a sufficient amount of flash memory cells exceeds the upper limit of tolerance,it means the end of the life of the solid state disk.As the storage bit density of flash media continues to rise,the number of program/erase operation that solid-state disks can withstand has dropped sharply,and extending the life of solid-state disks has become a research hotspot.Research shows that the higher voltage the cell is charged,the greater the wear on the flash memory cell suffers caused by the program/erase operation.Based on this feature,a low-wear data coding scheme for flash memory(named DREAM)is proposed.By making full use of the free area generated by data compression and the free area in the out-of-band(OOB),the flash life can be extended.DREAM uses the voltage combination of multiple flash memory cells instead of the traditional single flash memory cell voltage to represent data.By increasing the voltage of the flash memory cells in the free area and reducing the use of high wear cell voltages in other flash memory cells,the average wear causing by a single pair of program/erase operation can be reduced,which in turn increases the number of program/erase operations that the solid state disk can withstand,achieves a life extension effect.In addition,by introducing hot and cold data partitions,while reducing program/erase operation wear,the negative impact of DREAM on read performance can also be effectively controlled.The experimental results show that DREAM has a significant life extension effect,and the average life extension effect at different compression ratios reaches 23.91%;In addition,simulation results based on multiple workloads show that after using hot and cold data partitions,the average rate drop of solid disk throughput caused by DREAM does not exceed 2%,which is within the acceptable range.