We describe the design, implementation, and performance of MFS, a new file system designed to support efficient widearea collaboration. MFS is structured around the twin abstractions of lightweight sessions and snapshots, along with a highly configurable capability-based security architecture. Sessions simplify and clarify collaborative semantics. Snapshots allow atomic access to arbitrary collections of files, and allow sharing to be defined in a simple and expressive fashion.

MFS?s security architecture is a layered system that allows diverse usage scenarios. Pure capability-based access allows clients to access data without needing expensive public key or authentication servers, or complicated administration. However, MFS?s capabilities can also be watermarked, allowing a range of services to be added on a per-mount basis, up to and including traditional user authentication based on passwords or public keys.

Basing the system around the use of immutable snapshots enables the underlying system to use several performance optimizations aggressively. Performance results from our MFS prototype show that, far from adding overhead, the use of snapshots allows the system to perform comparably to NFS in the local-area case and significantly outperform existing systems in wide-area environments.