A Management Scheme of Multi-Level Retention-Time Queues for Improving the Endurance of Flash-Memory Storage Devices

Abstract

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.