Rosenthal first implemented stacking in SunOS 4.1 in the early 1990s[19]. A few other projects followed his, including further prototypes for extensible file systems in SunOS[22], and the Ficus layered file system[5,7]. Webber implemented file system interface extensions that allow user-level file servers[25]. Unfortunately, these implementations required modifications to either existing file systems or the rest of the kernel, limiting their portability significantly, and affecting the performance of native file systems. FiST achieves portability using a minimal stackable base file system, Basefs, which can be ported to another platform in 1-3 weeks. No changes need to be made to existing kernels or file systems, and there is no performance penalty for native file systems.
Newer operating systems, such as the HURD[4], Spring[13], and the Exokernel[10] have an extensible file system interface. The HURD is a set of servers running under the Mach 3.0 microkernel[1] that collectively provide a Unix-like environment. HURD translators are programs that can be attached to a pathname and perform specialized services when that pathname is accessed. Writing translators entails implementing a well defined file access interface and filling in stub operations for reading files, creating directories, listing directory contents, etc.
Sun Microsystems Laboratories built Spring, an object-oriented research operating system[13]. Spring was designed as a set of cooperating servers on top of a microkernel. It provides generic modules that offer services useful for a file system: caching, coherency, I/O, memory mapping, object naming, and security. Writing a file system for Spring involves defining the operations to be applied on the objects. Operations not defined are inherited from their parent object. One work that resulted from Spring is the Solaris MC (Multi-Computer) File System[12]. It borrowed the object-oriented interfaces from Spring and integrated them with the existing Solaris vnode interface to provide a distributed file system infrastructure through a special Proxy File System. Solaris MC provides all of Spring's benefits, while requiring little or no change to existing file systems; those can be ported gradually over time. Solaris MC was designed to perform well in a closely coupled cluster environment (not a general network) and requires high performance networks and nodes.
The Exokernel is an extensible operating system that comes with XN, a low-level in-kernel stable storage system[10]. XN allows users to describe the on-disk data structures and the methods to implement them (along with file system libraries called libFSes). The Exokernel requires significant porting work to each new platform, but then it can run many unmodified applications.
The main disadvantages of the HURD, Spring, and the Exokernel are that they are not portable enough, not sufficiently developed or stable, or they are not available for general use. In comparison, FiST provides portable stacking on widely available operating systems. Finally, none of the related extensible file systems come with a high-level language that developers can use to describe file systems.
High level languages have seldom been used to generate code for operating system components. FiST is the first major language to describe a large component of the operating system, the file system. Previous work in the area of operating system component languages includes a language to describe video device drivers[24].