# A Logic of File Systems

Muthian Sivathanu, Andrea C. Arpaci-Dusseau, Remzi H. Arpaci-Dusseau, Somesh Jha
Department of Computer Sciences , University of Wisconsin-Madison

### Abstract:

Years of innovation in file systems have been highly successful in improving their performance and functionality, but at the cost of complicating their interaction with the disk. A variety of techniques exist to ensure consistency and integrity of file system data, but the precise set of correctness guarantees provided by each technique is often unclear, making them hard to compare and reason about. The absence of a formal framework has hampered detailed verification of file system correctness.

We present a logical framework for modeling the interaction of a file system with the storage system, and show how to apply the logic to represent and prove correctness properties. We demonstrate that the logic provides three main benefits. First, it enables reasoning about existing file system mechanisms, enabling developers to employ aggressive performance optimizations without fear of compromising correctness. Second, the logic simplifies the introduction and adoption of new file system functionality by facilitating rigorous proof of their correctness. Finally, the logic helps reason about smart storage systems that track semantic information about the file system.

A key aspect of the logic is that it enables {\em incremental modeling}, significantly reducing the barrier to entry in terms of its actual use by file system designers. In general, we believe that our framework transforms the hitherto esoteric and error-prone art'' of file system design into a readily understandable and formally verifiable process.

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