In order to support variable persistence guarantees, we designed and implemented a new file system abstraction with two major extensions to conventional file system interfaces: a storage abstraction extension and a reconstruction extension. The storage abstraction extension allows programmers to specify the volatility of a file. MBFS stores the low-level volatility specification as part of the file's metadata and uses callbacks to maintain metadata and block consistency. The reconstruction extensions allows the system to automatically recreate most data that has been lost. The reconstruction extension is described in Section 6.
Although the MBFS logical storage hierarchy allows applications to specify the desired persistence, programmers will not use the system or will not make effective use of the system if the interface is too complex or difficult to select the correct persistence/performance tradeoff. While we expect many performance critical applications (for example, compilers, typesetters, web browsers, archival and compression tools, FTP) will be modified to optimize their performance using the new file system abstraction, we would like to offer MBFS's features to existing applications without the need to change them. In short, we would like to simplify the interface without sacrificing functionality. To that end, we use a multi-level design: a kernel-level raw interface that provides full control over the MBFS logical storage hierarchy and programmer-level interfaces that build on the raw level to provide an easy-to-use interface to applications. The following describes the raw kernel-level interface and two programmer-level interfaces, the category interface and the filename matching interface. Note that additional interface libraries (such as file mean-time-to-failure) could also be implemented on top of MBFS's raw interface. Each library is responsible for mapping their respective volatility specifications to the low-level storage details that the kernel interface expects. Although we describe our extensions as enhancements to the conventional file system abstraction, the extension could also be cleanly incorporated into emerging extensible file system standards such as the Scalable I/O Initiative's proposed PFS[8].