The advent of persistent memory (PM) into our everyday computing environment is now imminent. New programming models and algorithms based on these models are being developed for such systems. However, current models require programs to be rewritten with persistence related primitives such as clflush and clwb or at least recompiled so that persistent mechanisms can be automatically inserted. This is a burden to program developers. Furthermore, executing legacy programs as-is can lead to application and system inconsistencies as unexpected faults occur. In this article, we propose µSnap, an operating system support that is based on checkpointing that allows legacy applications to be executed as-is without compromising consistency in systems that deploy PM. We implement a prototype of µSnap in the Linux kernel version 4.3.3, and measure and quantify the effect of µSnap for a wide range of applications. We find that µSnap incurs overhead for application execution compared to applications run without any notion of persistency, but that the overhead can be controlled to be minimal by appropriately setting the checkpointing interval. We argue that the benefit for paying this small cost can be tremendous in the sense that one can transparently guarantee the consistency of all legacy software written under the traditional programming model.

The advent of Persistent Memory (PM), which is anticipated to have byte-addressable access latency in par with DRAM and yet nonvolatile, has stepped up interest in using PM as storage. Hence, PM storage targeted file systems are being developed under the premise that legacy file systems are suboptimal on memory bus attached PM-based storage. However, many years of time and effort are ingrained in legacy file systems that are now time-tested and mature. Simply scrapping them altogether may be unwarranted. In this paper, we look into how we can leverage the maturity ingrained in legacy file systems to the fullest, while, at the same time, reaping the high performance offered by PM. To this end, we first go through a thorough analysis of legacy Ext4 file systems, and compare it with NOVA, PMFS, and Ext4 with DAX extension, which are new PM file systems available in Linux. Based on these analyses, we then propose the Persistent Memory Adaptation Layer (PMAL) module that is lightweight (roughly 180 LoC) and can easily be integrated into legacy file systems to take advantage of PM storage. Using Ext4, we show that the performance of PMAL integrated Ext4 is in par with PM file systems for the Filebench and key-value store benchmarks.