System Administration Guide: Devices and File Systems

Mounting and Unmounting File Systems

Before you can access the files on a file system, you need to mount the file system. When you mount a file system, you attach that file system to a directory (mount point) and make it available to the system. The root (/) file system is always mounted. Any other file system can be connected or disconnected from the root (/) file system.

When you mount a file system, any files or directories in the underlying mount point directory are unavailable as long as the file system is mounted. These files are not permanently affected by the mounting process, and they become available again when the file system is unmounted. However, mount directories are typically empty, because you usually do not want to obscure existing files.

For example, the following figure shows a local file system, starting with a root (/) file system and the sbin, etc, and opt subdirectories.

Figure 15–1 Sample root (`/`) File System

Diagram shows sample root (/) file system with partial entries
from the sbin, etc, and opt directories listed.

To access a local file system from the /opt file system that contains a set of unbundled products, you must do the following:

First, you must create a directory to use as a mount point for the file system you want to mount, for example, /opt/unbundled.
Once the mount point is created, you can mount the file system (by using the mount command), which makes all of the files and directories in /opt/unbundled available, as shown in the following figure.

For step-by-step instructions on how to mount file systems, see Chapter 17, Mounting and Unmounting File Systems (Tasks).

Figure 15–2 Mounting a File System

Diagram shows mounting a file system on the /opt/unbundled mount
point with a listing of the newly accessible items in the /opt/unbundled directory.

The Mounted File System Table

Whenever you mount or unmount a file system, the /etc/mnttab (mount table) file is modified with the list of currently mounted file systems. You can display the contents of this file with the cat or more commands, but you cannot edit it. Here is an example of an /etc/mnttab file:

$ more /etc/mnttab
/dev/dsk/c0t0d0s0  /  ufs rw,intr,largefiles,onerror=panic,suid,dev=2200000 938557523
/proc   /proc   proc    dev=3180000     938557522
fd      /dev/fd fd      rw,suid,dev=3240000     938557524
mnttab  /etc/mnttab     mntfs   dev=3340000     938557526
swap    /var/run        tmpfs   dev=1   938557526
swap    /tmp    tmpfs   dev=2   938557529
/dev/dsk/c0t0d0s7 /export/home ufs rw,intr,largefiles,onerror=panic,suid,dev=2200007 ...
$

The Virtual File System Table

It would be a very time-consuming and error-prone task to manually mount file systems every time you wanted to access them. To avoid this problem, the virtual file system table (the /etc/vfstab file) provides a list of file systems and how to mount them.

The /etc/vfstab file provides two important features:

You can specify file systems to automatically mount when the system boots.
You can mount file systems by using only the mount point name, because the /etc/vfstab file contains the mapping between the mount point and the actual device slice name.

A default /etc/vfstab file is created when you install a system, depending on the selections you make when installing system software. However, you can edit the /etc/vfstab file on a system whenever you want. To add an entry, the main information you need to specify is the device where the file system resides, the name of the mount point, the type of the file system, whether you want the file system to mount automatically when the system boots (by using the mountall command), and any mount options.

The following is an example of an /etc/vfstab file. Comment lines begin with #. This example shows an /etc/vfstab file for a system with two disks (c0t0d0 and c0t3d0).

$ more /etc/vfstab
#device         device          mount           FS      fsck   mount  mount
#to mount       to fsck         point           type    pass   at boot options
/dev/dsk/c0t0d0s0 /dev/rdsk/c0t0d0s0 /          ufs     1       no      -
/proc           -               /proc           proc    -       no      -
/dev/dsk/c0t0d0s1 -                -            swap    -       no      -
swap            -               /tmp            tmpfs   -       yes     -
/dev/dsk/c0t0d0s6 /dev/rdsk/c0t0d0s6 /usr       ufs     2       no      -
/dev/dsk/c0t3d0s7 /dev/rdsk/c0t3d0s7 /test      ufs     2       yes     -
$

In the preceding example, the last entry specifies that a UFS file system on the /dev/dsk/c0t3d0s7 slice will be automatically mounted on the /test mount point when the system boots. Note that, for root (/) and /usr, the mount at boot field value is specified as no, because these file systems are mounted by the kernel as part of the boot sequence before the mountall command is run.

For descriptions of each of the /etc/vfstab fields and information on how to edit and use the file, see Chapter 17, Mounting and Unmounting File Systems (Tasks).

The NFS Environment

NFS is a distributed file system service that can be used to share resources (files or directories) from one system, typically a server, with other systems on the network. For example, you might want to share third-party applications or source files with users on other systems.

NFS makes the actual physical location of the resource irrelevant to the user. Instead of placing copies of commonly used files on every system, NFS allows you to place one copy on one system's disk and let all other systems access it from the network. Under NFS, remote files are virtually indistinguishable from local ones.

A system becomes an NFS server if it has resources to share on the network. A server keeps a list of currently shared resources and their access restrictions (such as read/write or read-only access).

When you share a resource, you make it available for mounting by remote systems.

You can share a resource in these ways:

By using the share or shareall command
By adding an entry to the /etc/dfs/dfstab (distributed file system table) file and rebooting the system

For information on how to share resources, see Chapter 17, Mounting and Unmounting File Systems (Tasks). For a complete description of NFS, see Chapter 14, Managing Network File Systems (Overview), in System Administration Guide: Resource Management and Network Services.

Automounting or AutoFS

You can mount NFS file system resources by using a client-side service called automounting (or AutoFS), which enables a system to automatically mount and unmount NFS resources whenever you access them. The resource remains mounted as long as you remain in the directory and are using a file. If the resource is not accessed for a certain period of time, it is automatically unmounted.

AutoFS provides the following features:

NFS resources don't need to be mounted when the system boots, which saves booting time.
Users don't need to know the root password to mount and unmount NFS resources.
Network traffic might be reduced, since NFS resources are only mounted when they are in use.

The AutoFS service is initialized by the automount utility, which runs automatically when a system is booted. The automountd daemon runs continuously and is responsible for the mounting and unmounting of the NFS file systems on an as-needed basis. By default, the /home file system is is mounted by the automount daemon.

With AutoFS, you can specify multiple servers to provide the same file system. This way, if one of the servers is down, AutoFS can try to mount from another machine.

For complete information on how to set up and administer AutoFS, see System Administration Guide: IP Services.