In recent years, NAND flash memory has been widely used in mobile devices, laptops, desktops, and data center storage systems due to its low-power consumption, high performance, high density, lightweight, shock resistance, and high-reliability natures. However, as the stacked layers and the storage density increase, flash memory also suffers from a higher raw bit error rate (RBER) and shorter lifetime. Observing that reliable cell states suffer from less data retention errors and program disturbance, we propose a differential evolution coding scheme to increase the probability of storing data in more reliable cell states, thereby reducing the RBER. We conducted the experiments over a development platform of SSD storage device with 3D-TLC charge trap (CT) NAND flash memory. The experimental results showed that the proposed differential evolution algorithm with asymmetric coding scheme could averagely reduce RBER by 48.88%, 65.45%, 52.61%, 61.99%, 80.19%, and 33.18% compared with baseline, asymmetric coding algorithm, asymmetric coding scheme with stripe-pattern elimination algorithm, UAC-$n$LC, word-line batch score modulation programming, and SCB schemes.

As flash memory technology has been scaled down to 1x nm and more bits can be stored in a cell, the storage density of flash memory has been significantly improved. However, these technical trends also severely hurt the programming speed and endurance of flash memory. The internal data retention time is the duration for which a flash cell can correctly hold data. By relaxing internal data retention time, both the page programming speed and the block endurance could be improved. However, the retention time of flash memory typically requires to last for several years according to the industrial standard. Thus a refreshment scheme is required to deal with the decreasing of retention time. In this article, we propose multi-level retention-time queues with a management scheme to meet the retention-time requirement for a reliable storage system. Observing that many data are overwritten in hours or days in real workloads, multiple retention-time queues could effectively separate data with different update frequencies. There are three challenge issues for a proper design: (1) Since access pattern might change from time to time, a technical issue is how to promote/demote data so that data could be maintained in the proper retention-time queue to minimize the refreshment overhead. (2) Another technical issue is how to refresh each retention-time queue in time to guarantee data integrity. (3) Since blocks resided in different retention-time queue would suffer from different level of wearing, the third technical issue is how to estimate wearing status of flash-memory blocks in an effective and efficient manner to achieve wear leveling. In our scheme, data allocator, multi-level refresh module, garbage collector, and wear leveler are introduced to deal with these technical issues. Based on our experimental results, not only endurance and performance but also energy consumption of the flash-memory storage system could be significantly improved by our scheme.

As flash memory technology has been scaled to 1xnm and more bits can be stored in a cell, the storage density of flash memory has been significantly improved. However, these technical trends also severely hurt the programming speed and endurance of flash memory. The internal data retention time is the duration for which a flash cell can correctly hold data. By relaxing internal data retention time, both the page programming speed and the block endurance could be improved. However, the retention time of flash memory typically requires to last for several years according to the industrial standard. Thus a refreshment scheme is required to deal with the decreasing of retention time. In this paper, we present a retention-time relaxation scheme to meet the retention-time requirement for a reliable storage system. Observing that many data are overwritten in hours or days in real workloads, multiple retention-time queues are maintained to store data with different update frequency separately. Since access pattern might change from time to time, one technical issue is how to promote/demote data so that data could be maintained in the proper retention-time queue to minimize the refreshment overhead. Another technical issue is how to efficiently and effectively refresh each retention-time queue in time to guarantee data integrity. In our scheme, data allocator and multi-level refresh module are proposed to deal with these technical issues. Based on the experimental results, not only endurance and performance but also energy consumption of the flash-memory storage system could be significantly improved by our scheme.