Improving Fairness for SSD Devices through DRAM Over-Provisioning Cache Management

Modern NVMe SSDs have been widely deployed in multi-tenant cloud computing environments or multi-programming systems. When multiple applications concurrently access one SSD hardware, unfairness within the shared SSD will slow down the application significantly and lead to a violation of service level objectives. However, traditional data cache management within SSDs mainly focuses on improving cache hit ratio, which causes data cache contention and sacrifices fairness among multiple applications. In this paper, we propose a DRAM-based Over-Provisioning (OP) cache management mechanism, named Justitia, to reduce data cache contention and improve fairness for modern SSDs. Justitia consists of two stages including Static-OP stage and Dynamic-OP stage. Through the novel OP mechanism in the two stages, Justitia reduces the max slowdown by $4.5\times$ on average. At the same time, Justitia increases fairness by $20.6\times$ and buffer hit ratio by $19.6\%$ averagely, compared with the traditional shared mechanism.