UNIX Filesystems: Evolution, Design, and Implementation

What's in a Filesystem?

At one time, filesystems were either disk based in which all files in the filesystem were held on a physical disk, or were RAM based. In the latter case, the filesystem only survived until the system was rebooted. However, the concepts and implementation are the same for both. Over the last 10 to 15 years a number of pseudo filesystems have been introduced, which to the user look like filesystems, but for which the implementation is considerably different due to the fact that they have no physical storage. Pseudo filesystems will be presented in more detail in Chapter 11. This chapter is primarily concerned with disk-based filesystems.

A UNIX filesystem is a collection of files and directories that has the following properties:

• It has a root directory (/) that contains other files and directories. Most disk-based filesystems will also contain a lost+found directory where orphaned files are stored when recovered following a system crash.
• Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.
• By convention, the root directory has an inode number of 2 and the lost+found directory has an inode number of 3. Inode numbers 0 and 1 are not used. File inode numbers can be seen by specifying the -i option to ls.
• It is self contained. There are no dependencies between one filesystem and any other.

A filesystem must be in a clean state before it can be mounted. If the system ...