How automount works

Networking Guide
Chapter 15, Configuring the NFS automounter

How automount works

Chapter 14, ``Configuring the Network File System (NFS)'', explains how to mount remote filesystems on an NFS client using the conventional NFS mounting approach whose major characteristics are:

Automatic mounting at boot time.
Manual mounting if mounting is required after boot time.

automount provides an alternative approach that mounts filesystems automatically, but not until they are needed.

Table 15-1 Filesystem mounting: conventional vs. automount

 -------------------------------------------------------------------
 Conventional method                automount method
 -------------------------------------------------------------------
 Mounts occur automatically and     Mounts occur automatically and
 transparently at boot time or      transparently when a user tries
 manually thereafter.               to access any files or
                                    directories under the designated
                                    mount point of the remote
                                    filesystem to be mounted.
 -------------------------------------------------------------------
 Mounts must be removed manually,   Mounts are removed automatically
 or during system reboot.           when the contents of the mounted
                                    filesystem have not been used
                                    for a specified period of time.
 -------------------------------------------------------------------
 To manually access mounts, users   Users have transparent mount
 must use the mount and umount      access; the mounts are
 commands. (In both conventional    automatically triggered when the
 and automount methods, all user    user uses a command like cd to
 mounts must be preconfigured by    access files or directories that
 the system administrator).         are actually on a remote
                                    filesystem.  (In both
                                    conventional and automount 
                                    methods, all user mounts must be
                                    preconfigured by the system
                                    administrator).
 -------------------------------------------------------------------
 New links can be added at any      New links cannot be manually
 time by the system                 added and made active without
 administrator.                     rebooting automount.
 -------------------------------------------------------------------
 Only one NFS server may be         One or more NFS servers may be
 designated as the source of a      designated as the source of a
 filesystem, so mounts fail if      filesystem. This redundancy
 that server is unavailable.        means that if one server is
                                    unavailable, the mount may be
                                    obtained from another.
 -------------------------------------------------------------------
 Service is primarily provided by   Service primarily provided by
 the mount(ADM) and umount          the automount(NADM) command.
 commands.
 -------------------------------------------------------------------
 Files used are                     Files used are a variety of maps
 /etc/default/filesys for           for configuring automatic
 configuring automatic mounts at    mounts; /etc/mnttab for a
 boot time; /etc/mnttab for a       listing of all the currently
 listing of all the currently       mounted filesystems.  (see ``The
 mounted filesystems.               mount table'' for more
                                    information).

NOTE: The differences between these two approaches apply only to the client side of NFS mounting. With both approaches, the server must be configured to export the filesystems the client wants to mount and be configured to let that client mount those filesystems.

In summary, the benefits of automount are:

Booting time is significantly reduced because no mounting is done at boot time.
Network access and efficiency are improved by reducing the number of permanently active mount points.
Failed mount requests can be reduced by designating alternate servers as the source of a filesystem.

The one possible drawback to automount is:

Adding new mounts manually requires rebooting, unlike the conventional method.

Therefore, if you know you will regularly need to add new mounts, you may want to consider the relative benefits and drawbacks of both systems for your needs.