[ Previous | Next | Table of Contents | Index | Library Home | Legal | Search ]

System Management Concepts: Operating System and Devices

Mounting Overview

Mounting makes file systems, files, directories, devices, and special files available for use at a particular location. It is the only way a file system is made accessible. The mount command instructs the operating system to attach a file system at a specified directory.

You can mount a file or directory if you have access to the file or directory being mounted and write permission for the mount point. Members of the system group can also perform device mounts (in which devices or file systems are mounted over directories) and the mounts described in the /etc/filesystems file. A user operating with root user authority can mount a file system arbitrarily by naming both the device and the directory on the command line. The /etc/filesystems file is used to define mounts to be automatic at system initialization. The mount command is used to mount after system startup.

Understanding Mount Points

A mount point is a directory or file at which a new file system, directory, or file is made accessible. To mount a file system or a directory, the mount point must be a directory; and to mount a file, the mount point must be a file.

Typically, a file system, directory, or file is mounted over an empty mount point, but that is not required. If the file or directory that serves as the mount point contains any data, that data is not accessible while it is mounted over by another file or directory. In effect, the mounted file or directory covers what was previously in that directory. The original directory or file that has been mounted over is accessible again once the mount over it is undone.

When a file system is mounted over a directory, the permissions of the root directory of the mounted file system take precedence over the permissions of the mount point. The one exception involves the .. (dot dot) parent directory entry in the mounted-over directory. In order for the operating system to access the new file system, the mount point parent directory information must be available.

For example, if the current working directory is /home/frank, the command cd .. changes the working directory to /home. If /home/frank directory is the root of a mounted file system, the operating system must find the parent directory information in the /home/frank directory in order for the cd .. command to succeed.

For any command that requires parent directory information in order to succeed, users must have search permission in the mounted-over directory. Failure of the mounted-over directory to grant search permission can have unpredictable results, especially since the mounted-over directory permissions are not visible. A common problem is failure of the pwd command. Without search permission in the mounted-over directory, the pwd command returns this message:

pwd: Permission denied 

This problem can be avoided by always setting the permissions of the mounted-over directory to at least 111.

Mounting File Systems, Directories, and Files

There are two types of mounts, a remote mount and a local mount. Remote mounts are done on a remote system on which data is transmitted over a telecommunication line. Remote file systems, such as Network File System (NFS), require that the files be exported before they can be mounted. Local mounts are mounts done on your local system.

Each file system is associated with a different device (logical volume). Before you can use a file system, it must be connected to the existing directory structure (either the root file system or to another file system that is already connected). The mount command makes this connection.

The same file system, directory, or file can be accessed by multiple paths. For example, if you have one database and several users using this database, it can be useful to have several mounts of the same database. Each mount should have its own name and password for tracking and job-separating purposes. This is accomplished by mounting the same file system on different mount points. For example, you can mount from /home/server/database to the mount point specified as /home/user1, /home/user2, and /home/user3:

/home/server/database        /home/user1
/home/server/database        /home/user2
/home/server/database        /home/user3

A file system, directory, or file can be made available to various users through the use of symbolic links. Symbolic links are created with the ln -s command. Linking multiple users to a central file ensures that all changes to the file are reflected each time a user accesses the file.

Controlling Automatic Mounts

Mounts can be set to occur automatically during system initialization. There are two types of automatic mounts. The first type consists of those mounts that are required to boot and run the system. These file systems are explicitly mounted by the boot process. The stanzas of such file systems in the /etc/filesystems file have mount = automatic. The second type of automatic mount is user-controlled. These file systems are mounted by the /etc/rc script when it issues the mount all command. The stanzas of user-controlled automatic mounts have mount = true in /etc/filesystems.

The /etc/filesystems file controls automatic mounts; they are done hierarchically, one mount point at a time. They can also be placed in a specific order that can be changed and rearranged.

The /etc/filesystems file is organized into stanzas, one for each mount. A stanza describes the attributes of the corresponding file system and how it is mounted. The system mounts file systems in the order they appear in the /etc/filesystems file. The following is an example of stanzas within the /etc/filesystems file:


You can edit the /etc/filesystems file to control the order in which mounts occur. If a mount is unsuccessful, any of the following mounts defined in the /etc/filesystems file continue to mount. For example, if the mount of the /home file system is unsuccessful, the mount for the /usr file system continues and be mounted. Mounts can be unsuccessful for reasons such as typographical errors, dependency, or a system problem.

The ln command, mount command, umount command.

The filesystems file centralizes file system characteristics.

[ Previous | Next | Table of Contents | Index | Library Home | Legal | Search ]