Professional Linux Kernel Architecture

Chapter 9

The Extended Filesystem Family

The structure and layout of the interfaces and data structures of the Virtual Filesystem discussed in Chapter 8 define a framework within which filesystem implementations must operate. However, this does not dictate that the same ideas, approaches, and concepts must be adopted by every filesystem when files are organized on block devices to store their contents permanently. Quite the opposite: Linux supports a wide variety of concepts including those that are easy to implement and understand but are not particularly powerful (e.g., the Minix filesystem); the proven Ext2 filesystem, which is used by millions; specific versions designed to support RAM- and ROM-based approaches; highly available cluster filesystems; and modern, tree-based filesystems with rapid restoration of consistency by means of transaction journals. No other operating system offers this versatility.

The techniques used differ considerably even though they can all be addressed—from both the user and kernel sides—via an identical interface, thanks to the virtual filesystem. Because of the large number of filesystems supported, every single implementation cannot be discussed here—not even briefly. Instead, this chapter focuses on the extended filesystem family, that is, the Ext2 and Ext3 filesystems. They illustrate the key concepts underlying the development of filesystems.

9.1 Introduction

Ext3 and Ext3 can be briefly characterized as follows:

• The Second Extended Filesystem ...