Exploring Process Variation Aware Wear Leveling Mechanism For Inodes Of NVM-based In-memory File System

With the development of emerging non-volatile memory(NVM),many NVM-based in-memory file systems are proposed to exploit the advanced characteristics of NVM.Since the limited write endurance of NVM is not fully considered,these file systems may cause non-uniform wear distribution on NVM after performing a large number of file operations,especially for the inode section.Endurance variation caused by NVM's process variation makes things worse since uniform write distribution will cause the weakest storage domain to be worn out much earlier.In this thesis,we discuss the wear problem for the inode section,and propose a wear leveling mechanism,called Contour,including process variation aware space management scheme and wear leveling mechanism for inodes.Based on the inode virtualization technology,inodes can be moved dynamically between and within domains to balance the wear rate of inode slots from different domains.The main contributions of this thesis include:(1)The existing single-linked list of inodes may cause unbalanced wear across different storage domains.We propose a process variation aware space management scheme to distribute the inodes evenly among the storage domains.(2)We propose a process variation aware wear leveling mechanism to balance the wear rate of different storage domains.First,dynamically migrate hot inodes to storage domains with higher endurance.Second,limit the number of writes to each storage domain in round to regularly balance the wear rate.(3)Slots in the same storage domain may have different wear rate.We design an algorithm to perform dynamic migration of inodes within storage domains.The best-fit algorithm can distribute writes between adjacent storage domains,thereby avoiding excessive wear on a single storage domain.We implement the prototype of Contour in the NVM-based file system SIMFS,and compile it into the Linux kernel.Extensive experiments were conducted using standard benchmarks including Filebench,FIO and My SQL.The experimental results show that under four different workloads,Contour's standard deviation of page wear rate is better than that of No WL and PCV by 644.8 times and 11.0 times,respectively.The average performance overhead of Contour is 3.85%.