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Chapter 33. Mounting file systems on demand
As a system administrator, you can configure file systems, such as NFS, to mount automatically on demand.
33.1. The autofs service
This section explains the benefits and basic concepts of the autofs
service, used to mount file systems on demand.
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.
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.
Additional resources
-
The
autofs(8)
man page.
33.2. The autofs configuration files
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
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 33.1. The /etc/auto.master file
The following is a sample line from /etc/auto.master
file:
/mnt/data /etc/auto.data
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
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 tono
. - 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 33.2. 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
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.
Additional resources
-
autofs(5)
man page -
autofs.conf(5)
man page -
auto.master(5)
man page -
/usr/share/doc/autofs/README.amd-maps
file
33.3. Configuring autofs mount points
This procedure describes how to configure on-demand mount points using the autofs
service.
Prerequisites
Install the
autofs
package:# yum install autofs
Start and enable the
autofs
service:# systemctl enable --now autofs
Procedure
-
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. - In the map file, fill in the mount point, options, and location fields as described in The autofs configuration files section.
- Register the map file in the master map file, as described in The autofs configuration files section.
Allow the service to re-read the configuration, so it can manage the newly configured
autofs
mount:# systemctl reload autofs.service
Try accessing content in the on-demand directory:
# ls automounted-directory
33.4. Automounting NFS server user home directories with autofs service
This procedure describes how to configure the autofs service to mount user home directories automatically.
Prerequisites
- The autofs package is installed.
- The autofs service is enabled and running.
Procedure
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
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/&
You can skip
fstype
parameter, as it isnfs
by default. For more information, seeautofs(5)
man page.Reload the
autofs
service:# systemctl reload autofs
33.5. Overriding or augmenting autofs site configuration files
It is sometimes useful to override site defaults for a specific mount point on a client system.
Example 33.3. 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
The
auto.master
file contains:+auto.master
The NIS
auto.master
map file contains:/home auto.home
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/&
The
autofs
configuration optionBROWSE_MODE
is set toyes
:BROWSE_MODE="yes"
-
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 33.4. 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
The
/etc/auto.home
map contains the entry:* host.example.com:/export/home/&
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 33.5. Augmenting auto.home with only selected entries
Alternatively, to augment the site-wide auto.home
map with just a few entries:
Create an
/etc/auto.home
file map, and in it put the new entries. At the end, include the NISauto.home
map. Then the/etc/auto.home
file map looks similar to:mydir someserver:/export/mydir +auto.home
With these NIS
auto.home
map conditions, listing the content of the/home
directory outputs:$ ls /home beth joe mydir
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.
33.6. Using LDAP to store automounter maps
This procedure configures 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 theautofs
package.
Procedure
-
To configure LDAP access, modify the
/etc/openldap/ldap.conf
file. Ensure that theBASE
,URI
, andschema
options are set appropriately for your site. 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 33.6. 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"
Ensure that all other schema entries are commented in the configuration. The
automountKey
attribute of therfc2307bis
schema replaces thecn
attribute of therfc2307
schema. Following is an example of an LDAP Data Interchange Format (LDIF) configuration:Example 33.7. 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/&
Additional resources
33.7. Using systemd.automount to mount a file system on demand with /etc/fstab
This procedure shows how to 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
Add desired fstab entry as documented in Chapter 30. Persistently mounting file systems. For example:
/dev/disk/by-id/da875760-edb9-4b82-99dc-5f4b1ff2e5f4 /mount/point xfs defaults 0 0
-
Add
x-systemd.automount
to the options field of entry created in the previous step. Load newly created units so that your system registers the new configuration:
# systemctl daemon-reload
Start the automount unit:
# systemctl start mount-point.automount
Verification
Check that
mount-point.automount
is running:# systemctl status mount-point.automount
Check that automounted directory has desired content:
# ls /mount/point
Additional resources
-
systemd.automount(5)
man page. -
systemd.mount(5)
man page. - Introduction to systemd.
33.8. Using systemd.automount to mount a file system on demand with a mount unit
This procedure shows how to 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
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
-
Create a unit file with the same name as the mount unit, but with extension
.automount
. Open the file and create an
[Automount]
section. Set theWhere=
option to the mount path:[Automount] Where=/mount/point [Install] WantedBy=multi-user.target
Load newly created units so that your system registers the new configuration:
# systemctl daemon-reload
Enable and start the automount unit instead:
# systemctl enable --now mount-point.automount
Verification
Check that
mount-point.automount
is running:# systemctl status mount-point.automount
Check that automounted directory has desired content:
# ls /mount/point
Additional resources
-
systemd.automount(5)
man page. -
systemd.mount(5)
man page. - Introduction to systemd.