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Chapter 19. Mounting file systems on demand

As a system administrator, you can configure file systems, such as NFS, to mount automatically on demand.

19.1. The autofs service
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The autofs service can mount and unmount file systems automatically (on-demand), therefore saving system resources. It can be used to mount file systems such as NFS, AFS, SMBFS, CIFS, and local file systems.

One drawback of permanent mounting using the /etc/fstab configuration is that, regardless of how infrequently a user accesses the mounted file system, the system must dedicate resources to keep the mounted file system in place. This might affect system performance when, for example, the system is maintaining NFS mounts to many systems at one time.

An alternative to /etc/fstab is to use the kernel-based autofs service. It consists of the following components:

  • A kernel module that implements a file system, and
  • A user-space service that performs all of the other functions.

19.2. The autofs configuration files
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This section describes the usage and syntax of configuration files used by the autofs service.

The master map file

The autofs service uses /etc/auto.master (master map) as its default primary configuration file. This can be changed to use another supported network source and name using the autofs configuration in the /etc/autofs.conf configuration file in conjunction with the Name Service Switch (NSS) mechanism.

All on-demand mount points must be configured in the master map. Mount point, host name, exported directory, and options can all be specified in a set of files (or other supported network sources) rather than configuring them manually for each host.

The master map file lists mount points controlled by autofs, and their corresponding configuration files or network sources known as automount maps. The format of the master map is as follows: 

mount-point  map-name  options
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The variables used in this format are:

mount-point
The autofs mount point; for example, /mnt/data.
map-file
The map source file, which contains a list of mount points and the file system location from which those mount points should be mounted.
options
If supplied, these apply to all entries in the given map, if they do not themselves have options specified.

Example 19.1. The /etc/auto.master file

The following is a sample line from /etc/auto.master file: 

/mnt/data  /etc/auto.data
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Map files

Map files configure the properties of individual on-demand mount points.

The automounter creates the directories if they do not exist. If the directories exist before the automounter was started, the automounter will not remove them when it exits. If a timeout is specified, the directory is automatically unmounted if the directory is not accessed for the timeout period.

The general format of maps is similar to the master map. However, the options field appears between the mount point and the location instead of at the end of the entry as in the master map: 

mount-point  options  location
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The variables used in this format are:

mount-point
This refers to the autofs mount point. This can be a single directory name for an indirect mount or the full path of the mount point for direct mounts. Each direct and indirect map entry key (mount-point) can be followed by a space separated list of offset directories (subdirectory names each beginning with /) making them what is known as a multi-mount entry.
options
When supplied, these options are appended to the master map entry options, if any, or used instead of the master map options if the configuration entry append_options is set to no.
location
This refers to the file system location such as a local file system path (preceded with the Sun map format escape character : for map names beginning with /), an NFS file system or other valid file system location.

Example 19.2. A map file

The following is a sample from a map file; for example, /etc/auto.misc: 

payroll  -fstype=nfs4  personnel:/exports/payroll
sales    -fstype=xfs   :/dev/hda4
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The first column in the map file indicates the autofs mount point: sales and payroll from the server called personnel. The second column indicates the options for the autofs mount. The third column indicates the source of the mount.

Following the given configuration, the autofs mount points will be /home/payroll and /home/sales. The -fstype= option is often omitted and is not needed if the file system is NFS, including mounts for NFSv4 if the system default is NFSv4 for NFS mounts.

Using the given configuration, if a process requires access to an autofs unmounted directory such as /home/payroll/2006/July.sxc, the autofs service automatically mounts the directory.

The amd map format

The autofs service recognizes map configuration in the amd format as well. This is useful if you want to reuse existing automounter configuration written for the am-utils service, which has been removed from Red Hat Enterprise Linux.

However, Red Hat recommends using the simpler autofs format described in the previous sections.

19.3. Configuring autofs mount points
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Configure on-demand mount points by using the autofs service.

Prerequisites

  • Install the autofs package: 

    # dnf install autofs
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  • Start and enable the autofs service: 

    # systemctl enable --now autofs
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Procedure

  1. Create a map file for the on-demand mount point, located at /etc/auto.identifier. Replace identifier with a name that identifies the mount point.
  2. In the map file, enter the mount point, options, and location fields as described in The autofs configuration files section.
  3. Register the map file in the master map file, as described in The autofs configuration files section.

  4. Allow the service to re-read the configuration, so it can manage the newly configured autofs mount: 

    # systemctl reload autofs.service
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  5. Try accessing content in the on-demand directory: 

    # ls automounted-directory
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Configure the autofs service to mount user home directories automatically.

Prerequisites

  • The autofs package is installed.
  • The autofs service is enabled and running.

Procedure

  1. Specify the mount point and location of the map file by editing the /etc/auto.master file on a server on which you need to mount user home directories. To do so, add the following line into the /etc/auto.master file: 

    /home /etc/auto.home
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  2. Create a map file with the name of /etc/auto.home on a server on which you need to mount user home directories, and edit the file with the following parameters: 

    * -fstype=nfs,rw,sync host.example.com:/home/&
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    You can skip fstype parameter, as it is nfs by default. For more information, see autofs(5) man page on your system.

  3. Reload the autofs service: 

    # systemctl reload autofs
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It is sometimes useful to override site defaults for a specific mount point on a client system.

Example 19.3. Initial conditions

For example, consider the following conditions:

  • Automounter maps are stored in NIS and the /etc/nsswitch.conf file has the following directive: 

    automount:    files nis
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  • The auto.master file contains: 

    +auto.master
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  • The NIS auto.master map file contains: 

    /home auto.home
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  • The NIS auto.home map contains: 

    beth    fileserver.example.com:/export/home/beth
joe     fileserver.example.com:/export/home/joe
*       fileserver.example.com:/export/home/&
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  • The autofs configuration option BROWSE_MODE is set to yes: 

    BROWSE_MODE="yes"
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  • The file map /etc/auto.home does not exist.

Procedure

This section describes the examples of mounting home directories from a different server and augmenting auto.home with only selected entries.

Example 19.4. Mounting home directories from a different server

Given the preceding conditions, let’s assume that the client system needs to override the NIS map auto.home and mount home directories from a different server.

  • In this case, the client needs to use the following /etc/auto.master map: 

    /home ­/etc/auto.home
+auto.master
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  • The /etc/auto.home map contains the entry: 

    *    host.example.com:/export/home/&
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Because the automounter only processes the first occurrence of a mount point, the /home directory contains the content of /etc/auto.home instead of the NIS auto.home map.

Example 19.5. Augmenting auto.home with only selected entries

Alternatively, to augment the site-wide auto.home map with just a few entries:

  1. Create an /etc/auto.home file map, and in it put the new entries. At the end, include the NIS auto.home map. Then the /etc/auto.home file map looks similar to: 

    mydir someserver:/export/mydir
+auto.home
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  2. With these NIS auto.home map conditions, listing the content of the /home directory outputs: 

    $ ls /home

beth joe mydir
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This last example works as expected because autofs does not include the contents of a file map of the same name as the one it is reading. As such, autofs moves on to the next map source in the nsswitch configuration.

19.6. Using LDAP to store automounter maps
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Configure autofs to store automounter maps in LDAP configuration rather than in autofs map files.

Prerequisites

  • LDAP client libraries must be installed on all systems configured to retrieve automounter maps from LDAP. On Red Hat Enterprise Linux, the openldap package should be installed automatically as a dependency of the autofs package.

Procedure

  1. To configure LDAP access, modify the /etc/openldap/ldap.conf file. Ensure that the BASE, URI, and schema options are set appropriately for your site.

  2. The most recently established schema for storing automount maps in LDAP is described by the rfc2307bis draft. To use this schema, set it in the /etc/autofs.conf configuration file by removing the comment characters from the schema definition. For example:

    Example 19.6. Setting autofs configuration

    DEFAULT_MAP_OBJECT_CLASS="automountMap"
DEFAULT_ENTRY_OBJECT_CLASS="automount"
DEFAULT_MAP_ATTRIBUTE="automountMapName"
DEFAULT_ENTRY_ATTRIBUTE="automountKey"
DEFAULT_VALUE_ATTRIBUTE="automountInformation"
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  3. Ensure that all other schema entries are commented in the configuration. The automountKey attribute of the rfc2307bis schema replaces the cn attribute of the rfc2307 schema. Following is an example of an LDAP Data Interchange Format (LDIF) configuration:

    Example 19.7. LDIF Configuration

    # auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: top
objectClass: automountMap
automountMapName: auto.master

# /home, auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: automount
automountKey: /home
automountInformation: auto.home

# auto.home, example.com
dn: automountMapName=auto.home,dc=example,dc=com
objectClass: automountMap
automountMapName: auto.home

# foo, auto.home, example.com
dn: automountKey=foo,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: foo
automountInformation: filer.example.com:/export/foo

# /, auto.home, example.com
dn: automountKey=/,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: /
automountInformation: filer.example.com:/export/&
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Mount a file system on demand using the automount systemd units when mount point is defined in /etc/fstab. You have to add an automount unit for each mount and enable it.

Procedure

  1. Add desired fstab entry as documented in Persistently mounting file systems. For example: 

    /dev/disk/by-id/da875760-edb9-4b82-99dc-5f4b1ff2e5f4  /mount/point  xfs  defaults  0 0
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  2. Add x-systemd.automount to the options field of entry created in the previous step.

  3. Load newly created units so that your system registers the new configuration: 

    # systemctl daemon-reload
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  4. Start the automount unit: 

    # systemctl start mount-point.automount
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Verification

  1. Check that mount-point.automount is running: 

    # systemctl status mount-point.automount
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  2. Check that automounted directory has desired content: 

    # ls /mount/point
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Mount a file system on-demand using the automount systemd units when mount point is defined by a mount unit. You have to add an automount unit for each mount and enable it.

Procedure

  1. Create a mount unit. For example: 

    mount-point.mount
[Mount]
What=/dev/disk/by-uuid/f5755511-a714-44c1-a123-cfde0e4ac688
Where=/mount/point
Type=xfs
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  2. Create a unit file with the same name as the mount unit, but with extension .automount.

  3. Open the file and create an [Automount] section. Set the Where= option to the mount path: 

    [Automount]
Where=/mount/point
[Install]
WantedBy=multi-user.target
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  4. Load newly created units so that your system registers the new configuration: 

    # systemctl daemon-reload
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  5. Enable and start the automount unit instead: 

    # systemctl enable --now mount-point.automount
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Verification

  1. Check that mount-point.automount is running: 

    # systemctl status mount-point.automount
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  2. Check that automounted directory has desired content: 

    # ls /mount/point
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