Studies on Memory and Storage Management for Non-volatile Memory Systems

Abstract

Recently, high-speed non-volatile storage technologies such as PCM (Phase Change Memory) emerge and there is a bright prospect that PCM will be adopted in the design of computer systems. PCM is considered to be used as a part of DRAM-like memory since it is byte-addressable and provides a comparable access speed to DRAM. Furthermore, the rapid enhancement of micro-fabrication processes and multi-level cell (MLC) technologies leads to high capacity of PCM and allows it to be used as disk-like secondary storage as well. In this dissertation, we discuss several important issues in exploiting PCM in the memory and storage hierarchies of computer systems and present new designs of operating system components to use PCM efficiently. We first present an efficient memory management scheme when high-speed non-volatile memory such as PCM is adopted as secondary storage. Our proposed scheme allows interactive jobs to use a part of real-time tasks memory area during the slack time of the real-time task. The memory area is then restored into the original state of the real-time task before it is activated. This scheme is feasible because a page loading time is short and predictable when high speed non-volatile storage is used. We show that the page faults of interactive jobs are significantly reduced by 8.68% and 69.11% when the slack time changes from 20ms and 300ms. Secondly, we present an in-memory journaling scheme for PCM based main memory systems. As PCM is non-volatile, it is possible for main memory to retain data without power. Motivated by this, we investigate an efficient way to journal data in main memory instead of secondary storage. We have implemented the prototype of the proposed scheme on Linux and show our scheme improves performance by up to 43.7% than ext3 for multiple benchmarks. In addition, we redesign a journaling file system and a snapshot file system targeting on high-speed and byte-addressable storage backed by PCM. Considering the advantages and disadvantages of PCM based storage systems, our proposed file system improves performance and reliability, as well as endurance of storage. Lastly, we present an efficient buffer cache management scheme for high-speed secondary storage that performs nearly as fast as main memory. Since the optimistic access time of PCM is expected to be almost identical to that of DRAM, the traditional buffer cache may not be effective for PCM storage. We analyze the cost and benefit of caching when the speed gap of cache and storage becomes small and find the condition for caching to be beneficial by characterizing I/O workload patterns. In each issue, we provide a comprehensive analysis of ex-isting techniques and validate our scheme by implementation and trace-driven si-mulations.