A protected block device for Persistent Memory

Abstract

Persistent Memory (PM) technologies, such as Phase Change Memory, STT-RAM, and memristors, are receiving increasingly high interest in academia and industry. PM provides many attractive features, such as DRAM-like speed and storage-like persistence. Yet, because it draws a blurry line between memory and storage, neither a memory- or storage-based model is a natural fit. Best integrating PM into existing systems has become challenging and is now a top priority for many. In this paper we share our initial approach to integrating PM into computer systems, with minimal impact to the core operating system. By adopting a hybrid storage model, all of our changes are confined to a block storage driver, called PMBD, which directly accesses PM attached to the memory bus and exposes a logical block I/O interface to users. We explore the design space by examining a variety of options to achieve performance, protection from stray writes, ordered persistence, and compatibility for legacy file systems and applications. All told, we find that by using a combination of existing OS mechanisms (per-core page table mappings, non-temporal store instructions, memory fences, and I/O barriers), we are able to achieve each of these goals with small performance overhead for both micro-benchmarks and real world applications (e.g., file server and database workloads). Our experience suggests that determining the right combination of existing platform and OS mechanisms is a non-trivial exercise. In this paper, we share both our failed and successful attempts. The final solution that we propose represents an evolution of our initial approach. We have also open-sourced our software prototype with all attempted design options to encourage further research in this area.