Understanding the Linux Kernel, 3rd Edition

The Role of the Virtual Filesystem (VFS)

The Virtual Filesystem (also known as Virtual Filesystem Switch or VFS) is a kernel software layer that handles all system calls related to a standard Unix filesystem. Its main strength is providing a common interface to several kinds of filesystems.

For instance, let’s assume that a user issues the shell command:

    $ cp /floppy/TEST /tmp/test

where /floppy is the mount point of an MS-DOS diskette and /tmp is a normal Second Extended Filesystem (Ext2) directory. The VFS is an abstraction layer between the application program and the filesystem implementations (see Figure 12-1(a)). Therefore, the cp program is not required to know the filesystem types of /floppy/TEST and /tmp/test. Instead, cp interacts with the VFS by means of generic system calls known to anyone who has done Unix programming (see the section "File-Handling System Calls" in Chapter 1); the code executed by cp is shown in Figure 12-1(b).

Figure 12-1. VFS role in a simple file copy operation

Filesystems supported by the VFS may be grouped into three main classes:

Disk-based filesystems

These manage memory space available in a local disk or in some other device that emulates a disk (such as a USB flash drive). Some of the well-known disk-based filesystems supported by the VFS are:

• Filesystems for Linux such as the widely used Second Extended Filesystem (Ext2), the recent Third Extended ...