Understanding the Linux Kernel, 3rd Edition

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 filesystem types in the same way other Unix variants do, through a concept called the Virtual Filesystem.

The idea behind the Virtual Filesystem is to put a wide range of information in the kernel to represent many different types of filesystems ; there is a field or function to support each operation provided by all real filesystems supported by Linux. For each read, write, or other function called, the kernel substitutes the actual function that supports a native Linux filesystem, the NTFS filesystem, or whatever other filesystem the file is on.

This chapter discusses the aims, structure, and 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 are covered in Chapter 13, while pipes are discussed in Chapter 19. To show how a real filesystem works, Chapter 18 covers the Second Extended Filesystem that appears on nearly all Linux systems.