Directory Structure

Up to this point, we have been discussing "a file system." In reality, systems may have zero or more file systems, and the file systems may be of varying types. For example, a typical Solaris system may have a few UFS file systems, a VFS file system, and some NFS file systems. The file systems of computers, then, can be extensive. Some systems store millions of files on terabytes of disk. To manage all these data, we need to organize them. This organization involves the use of directories. In this section, we explore the topic of directory structure. First, though, we explain some basic features of storage structure.

1. Storage Structure
A disk (or any storage device that is large enough) can be used in its entirety for a file system. Sometimes, though, it is desirable to place multiple file systems on a disk or to use parts of a disk for a file system and other parts for other things, such as swap space or unformatted (raw) disk space. These parts are known variously as partitions, slices, or (in the IBM world) minidisks. A file system can be created on each of these parts of the disk. As we shall see in the next chapter, the parts can also be combined to form larger structures known as volumes, and file systems can be created on these as well. For now, for clarity, we simply refer to a chunk of storage that holds a file system as a volume. Each volume can be thought of as a virtual disk. Volumes can also store multiple operating systems, allowing a system to boot and run more than one. Each volume that contains a file system must also contain information about the files in the system. This information is kept in entries in a device directory or volume table of contents. The device directory (more commonly known simply as a directory) records information—such as name, location, size, and type—for all files on that volume. Figure 10.6 shows a typical file-system organization.

2. Directory Overview

The directory can be viewed as a symbol table that translates file names into their directory entries. If we take such a view, we see that the directory itself can be organized in many ways. We want to be able to insert entries, to delete entries, to search for a named entry, and to list all the entries in the directory. In this section, we examine several schemes for defining the logical structure of the directory system. When considering a particular directory structure, we need to keep in mind the operations that are to be performed on a directory: 

• Search for a file. We need to be able to search a directory structure to find the entry for a particular file. Since files have symbolic names and similar names may indicate a relationship between files, we may want to be able to find all files whose names match a particular pattern. 
• Create a file. New files need to be created and added to the directory.
•  Delete a file. When a file is no longer needed, we want to be able to remove it from the directory.
•  List a directory. We need to be able to list the files in a directory and the contents of the directory entry for each file in the list.
•  Rename a file. Because the name of a file represents its contents to its users, we must be able to change the name when the contents or use of the file changes. Renaming a file may also allow its position within the directory structure to be changed.
•  Traverse the file system. We may wish to access every directory and every file within a directory structure. For reliability, it is a good idea to save the contents and structure of the entire file system at regular intervals. Often, we do this by copying all files to magnetic tape. This technique provides a backup copy in case of system failure. In addition, if a file is no longer in use., the file can be copied to tape and the disk space of that file released for reuse by another file.

In the following sections, we describe the most common schemes for defining
the logical structure of a directory.

3. Single-Level Directory

A single-level directory has significant limitations, however, when the number of files increases or when the system has more than one user. Since all files are in the same directory, they must have unique names. If two users call their data file test, then the unique-name rule is violated. For example, in one programming class, 23 students called the program for their second assignment progl; another 11 called i\ assign!. Although file names are generally selected to reflect the content of the file, they are often limited in length, complicating the

task of making file names unique. The MS-DOS operating system allows only 11-character file names; UNIX, in contrast, allows 255 characters.

Even a single user on a single-level directory may find it difficult to remember the names of all the files as the number of files increases. It is not uncommon for a user to have hundreds of files on one computer system and an equal number of additional files on another system. Keeping track of so many

files is a daunting task.