Research On Wear-Leveling Algorithm Of Hybrid SSD

Since the erasable times of NAND flash memories are limited and the unbalanced erasure can reduce its reliability,it is critical to design a wear leveling algorithm with adaptive equalizing erase times to extend the life length of it.Mixed SSDs are usually composed of SLC(Single-Level Cell)and MLC(Multi-Level Cell)flash memory.Cold and hot data allocation between SLC and MLC flash memory,wear equalization between flash memories and internal wear equalization of flash memories are the key issues in the design of flash conversion layer of hybrid solid-state hard disk.The following works have been done corresponding to these issues:1)Based on the respective characteristics of SLC and MLC flash memory,SLC is suitable for processing small and hot data,while MLC is suitable for processing large and cold data.Since hot data will be frequently read and written,if hot data is deployed in SLC flash memory as much as possible,the overall data response time will be greatly reduced.This paper designs a kind of splitter whose threshold changes dynamically with the current read/write request size.Experiments show that the dynamic threshold algorithm in this paper can identify hot and cold data well.2)For the problems of unbalanced internal wear-leveling and low flash utilization in mixed solid-state systems,this paper uses physical page circular queues to balance the wear level,and manages the hot data in the SLC by controlling the length of the cyclic queues,which will increase the utilization of SLC flash memory.Generally speaking,the endurance of flash memory is 10 to 20 times longer than that of MLC and the problem of wear degree non-synchronization between the SLC and MLC flash memory in hybrid SSDs is prone to happen.In order to solve this problem,a flow controller is designed to redistribute the cold and hot data selectively through the wear equalization degree of the two and the real-time heat of the data,which can effectively balance the wear degree of the two and reduce the migration times of data between flash memories.The experimental results show that the above method can effectively improve the wear equalization degree within the SLC,the utilization rate of the flash memory and wear equalization degree between the SLC and the MLC flash memory.3)In view of the problem of internal wear equalization in MLC,this paper proposes a static wear algorithm and a dynamic wear equalization algorithm based on the combination of heat of logical pages and heat of physical blocks,and redefines the heat of data.The algorithm takes into account not only the physical page but also the heat of the logical page and the update time interval of the logical page.For the identification of hot and cold blocks,the physical properties of flash memory are taken into account to solve the problem of uneven erasure of physical blocks(some blocks are almost not erased,but some blocks are erased repeatedly).Experimental results show that the proposed algorithm can greatly improve the internal wear equalization of MLC flash memory and improve the service life of flash memory.