Section 14.1. File System Framework

14.1. File System Framework

Solaris OS includes a framework, the virtual file system framework, under which multiple file system types are implemented. Earlier implementations of UNIX used a single file system type for all of the mounted file systems, typically, the UFS file system from BSD UNIX. The virtual file system framework was developed to allow Sun's distributed computing file system (NFS) to coexist with the UFS file system in SunOS 2.0; it became a standard part of System V in SVR4 and Solaris OS. We can categorize Solaris file systems into the following types:

• Storage-based. Regular file systems that provide facilities for persistent storage and management of data. The Solaris UFS and PC/DOS file systems are examples.

• Network file systems. File systems that provide files that are accessible in a local directory structure but are stored on a remote network server; for example, NFS.

• Pseudo file systems. File systems that present various abstractions as files in a file system. The /proc pseudo file system represents the address space of a process as a series of files.

The framework provides a single set of well-defined interfaces that are file system independent; the implementation details of each file system are hidden behind these interfaces. Two key objects represent these interfaces: the virtual file, or vnode, and the virtual file system, or vfs objects. The vnode interfaces implement file-related functions, and the vfs interfaces implement file system management functions. The vnode and vfs interfaces direct functions to specific file systems, depending on the type of file system being operated on. Figure 14.1 shows the file system layers. File-related functions are initiated through a system call or from another kernel subsystem and are directed to the appropriate file system by the vnode/vfs layer.

Figure 14.1. Solaris File System Framework