Understanding the Linux Kernel

Chapter 12. The Virtual Filesystem

One of Linux's keys to success is its ability to coexist comfortably with other systems. You can transparently mount disks or partitions that host file formats used by Windows, other Unix systems, or even systems with tiny market shares like the Amiga. Linux manages to support multiple disk types in the same way other Unix variants do, through a concept called the Virtual Filesystem.

The idea behind the Virtual Filesystem is that the internal objects representing files and filesystems in kernel memory embody a wide range of information; there is a field or function to support any operation provided by any real filesystem supported by Linux. For each read, write, or other function called, the kernel substitutes the actual function that supports a native Linux filesystem, the NT filesystem, or whatever other filesystem the file is on.

This chapter discusses the aims, the structure, and the implementation of Linux's Virtual Filesystem. It focuses on three of the five standard Unix file types, namely, regular files, directories, and symbolic links. Device files will be covered in Chapter 13, while pipes will be discussed in Chapter 18. To show how a real filesystem works, Chapter 17, covers the Second Extended Filesystem that appears on nearly all Linux systems.