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.

中文翻译：

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一种提高闪存存储设备耐久性的多级保留时间队列管理方案

由于闪存技术已缩小到 1x nm，并且可以在一个单元中存储更多位，因此闪存的存储密度得到了显着提高。然而，这些技术趋势也严重损害了闪存的编程速度和耐久性。内部数据保留时间是闪存单元可以正确保存数据的持续时间。通过放宽内部数据保留时间，可以提高页面编程速度和块耐用性。然而，根据工业标准，闪存的保留时间通常需要持续数年。因此需要一个更新方案来处理保留时间的减少。在本文中，我们提出了具有管理方案的多级保留时间队列，以满足可靠存储系统的保留时间要求。观察到在实际工作负载中许多数据在数小时或数天内被覆盖，多个保留时间队列可以有效地分离具有不同更新频率的数据。正确设计存在三个挑战问题：(1) 由于访问模式可能会不时发生变化，因此技术问题是如何提升/降级数据，以便将数据维护在适当的保留时间队列中以最小化刷新高架。(2) 另一个技术问题是如何及时刷新每个保留时间队列以保证数据的完整性。(3) 由于块驻留在不同的保留时间队列中会受到不同程度的磨损，第三个技术问题是如何有效地估计闪存块的磨损状态以实现磨损均衡。在我们的方案中，数据分配器，多级刷新模块，垃圾收集器和磨损均衡器被引入来处理这些技术问题。根据我们的实验结果，我们的方案不仅可以显着提高闪存存储系统的耐用性和性能，还可以显着提高其能耗。