Performance and Protection in the ZoFS User-space NVM File System

Non-volatile memory (NVM) can be directly accessed in user space without going through the kernel. This encourages several recent studies on building user-space NVM file systems. However, for the sake of file system protection, none of the existing file systems grant user-space file system libraries with direct control over both metadata and data of the NVM, leaving fast NVM resources underexploited. Based on the observation that applications tend to group files with similar access permissions within the same directory and permission changes are rare operations, this paper proposes a new abstraction called coffer, which is a collection of isolated NVM resources, and show its merits on building a performant and protected NVM file system in user space. The key idea is to separate NVM protection from management via coffers so that user-space libraries can take full control of NVM within a coffer while the kernel guarantees strict isolation among coffers. Based on coffers, we build an NVM file system architecture to bring the high performance of NVM to unmodified dynamically linked applications and facilitate the development of performant and flexible user-space NVM file system libraries. With an example file system called ZoFS, we show that user-space file systems built upon coffers can outperform existing NVM file systems in both benchmarks and real-world applications.