Understanding the Linux Kernel

Chapter 17. The Ext2 Filesystem

In this chapter, we finish our extensive discussion of I/O and filesystems by taking a look at the details the kernel has to take care of when interacting with a particular filesystem. Since the Second Extended Filesystem (Ext2) is native to Linux and is used on virtually every Linux system, it made a natural choice for this discussion. Furthermore, Ext2 illustrates a lot of good practices in its support for modern filesystem features with fast performance. To be sure, other filesystems will embody new and interesting requirements, because they are designed for other operating systems, but we cannot examine the oddities of various filesystems and platforms in this book.

After introducing Ext2 in Section 17.1, we describe the data structures needed, just as in other chapters. Since we are looking at a particular way to store data on a disk, we have to consider two versions of data structures: Section 17.2 shows the data structures stored by Ext2 on the disk, while Section 17.3 shows how they are duplicated in memory.

Then we get to the operations performed on the filesystem. In Section 17.4, we discuss how Ext2 is created in a disk partition. The next sections describe the kernel activities performed whenever the disk is used. Most of these are relatively low-level activities dealing with the allocation of disk space to inodes and data blocks. Then we'll discuss how Ext2 regular files are read and written.