Garbage Collection Strategy Research Based On Mixed Cache Mechanism

NAND Flash-based SSDs have become the mainstream storage medium for secondar-y storage systems because of their many advantages such as good read/write performa-nce and high reliability.Because of the pre-write erasure feature of NAND Flash,it cannot be overwritten in-place during data update.In order to improve the write perfor-mance,it usually adopts an off-site update method,but this method will generate mor-e invalid old version data.To make more free space,usually,garbage collection is an effective mechanism used by NAND FLASH to regain invalid data blocks generated by remote updates.However,high-cost garbage collection brings new problems.Garb-age collection is an operation which is time-consuming and it will cause the loss of flash memory life.According to the analysis of the issues such as response delay,low resource utilization and high data migration overhead caused by garbage collection,a garbage collection strategy MC-GC based on mixed cache has been proposed in this thesis.The general work of MC-GC can be divided into the following parts:First,when garbage collection is performed on a chip,other chips mounted on the same channel cannot perform data transmission,resulting in low chip utilization and system response delay.Based on the idea of caching,this paper proposes a write-page data write-back delay strategy.The selective cache partially blocks the request data page,and performs data writing and garbage collection in parallel.Second,in view of the system response delay and write amplification caused by data migration to flash in the garbage collection process,this paper designs a data migration scheme based on hybrid cache.For the pre-recovery block,a new recovery cost function is constructed.And using the cache to carry out the de-redundant design of the data page migration,it effectively reduces the data migration overhead of garbage collection.Finally,for the data temporarily stored in the hybrid cache,a mixed cache management module based on page popularity is designed,in which the colder page data is written back to the flash memory preferentially,and the I/O allocation method for writing back the cached data page is performed.This paper considers the channel busyness and the number of writes to perform I/O allocation,thereby reducing the overall system response delay and the loss of flash memory life.In order to take full advantage of the advantages of PCM and DRAM,we design a paging scheme based on garbage collection to ensure more writes to DRAM while ensuring more intensive reads of data pages in PCM.Experimental results show that MC-GC has a good improvement in write performance,wear leveling,write times,and data migration overhead.