In this paper we presented a new file system abstraction for memory-based file management systems. The abstraction is unique in the fact that it allows applications or users to control file persistence on a per-file basis. Applications that can tolerate weak persistence can achieve substantial performance improvements by selecting memory-based storage. The new abstraction has two major extensions to conventional file systems abstractions. First, applications can define a file's persistence requirements. Second, applications can specify rules to reconstruct file data in the unlikely event that it is lost. Analysis of current file systems indicates that a large percentage of write traffic can benefit from weak persistence. To support large amounts of data with weak persistence guarantees, we developed a high-speed loosely-coupled memory storage system that utilizes the idle memory in a network of workstations. Our prototype implementation of the MBFS system running on Solaris and Linux systems shows applications speedups of an order of magnitude or more.