The Modify-on-Access File System:
An Extensible Linux File System¹

Richard Kendall and Vincent W. Freeh
{rkendall,vin}@cse.nd.edu
Department of Computer Science and Engineering
University of Notre Dame
Notre Dame, IN  46556

Abstract:

This paper introduces the Modify-on-Access (MonA) file system, an active file system that cooperates with processes to accomplish tasks. MonA provides a framework for dynamically pushing and popping modular computations, transformations, onto data streams during file accesses. As a result, the MonA file system simplifies system software and application programs by making critical read and write paths more optimal.

The MonA file system is implemented in Linux and supports extensions to the kernel as well as user-defined extensions that safely execute in user space. As a result, the file system is efficient and flexible without endangering system security. A kernel-resident identity transformation (functionally equivalent to the base file system) adds an overhead of 3-5% during a file access. A transformation that executes within the kernel is roughly 60 times faster than a user-space equivalent. However, MonA can still pass a page of data (4 KB) through a user-space transformation in under 1 ms, which is sufficient for many applications.