| File systems often need to evolve and require many changes to support new features. Traditional file-system development is difficult because most of the work is done in the kernel—a hostile development environment where progress is slow, debugging is difficult, and simple mistakes can crash systems. Kernel work also requires deep understanding of system internals, resulting in developers spending a lot of time becoming familiar with the system's details. Furthermore, any file system written for one system requires significant effort to port to another system. Stackable file systems promise to ease the development of file systems by offering a mechanism for incremental development building on existing file systems. Unfortunately, existing stacking methods often require writing complex low-level kernel code that is specific to a single operating system platform and also difficult to port.; We propose a new language, FiST, to describe stackable file systems. FiST uses operations common to file-system interfaces and familiar to user-level developers: creating a directory, reading a file, removing a file, listing the contents of a directory, etc. From a single description, FiST's compiler produces file-system modules for multiple platforms. FiST does that with the assistance of platform-specific stackable templates. The templates handle many of the internal details of operating systems, and free developers from dealing with these internals. The templates support many features: data copying and file name copying useful for applications that want to modify them; size-changing file systems such as compression; fan-out for access to multiple file systems from one layer; and more. The FiST language compiler uses the templates as a basis for producing code for a new file system, by inserting, removing, or modifying code in the templates.; This dissertation describes the design, implementation, and evaluation of FiST. Our thesis is that it is possible to extend file system functionality in a portable way without changing existing kernels. This is possible because the FiST language uses file-system functions that are common across many systems, while the templates execute in-kernel operating systems specific functions unchanged. We built several file systems using FiST on Solaris, FreeBSD, and Linux. Our experiences with these file systems show the following benefits: average code size is reduced ten times as compared to writing code given another null-layer stackable file system; average development time is reduced seven times compared to writing using another null-layer stackable file system; performance overhead of stacking is only 1–2% per layer. |
