How to mount partition using systemd unit file? [SOLVED]


Written By - Deepak Prasad

Systemd, the modern and powerful system and service manager for Linux, has become an essential component of many distributions, replacing traditional init systems. With its extensive capabilities and integration with numerous system processes, systemd simplifies the management of services, daemons, and system resources, including the mounting and unmounting of partitions.

In this tutorial, we will explore how to mount partitions using systemd unit files. This approach provides a reliable and efficient way to manage storage devices and their mount points, especially during system boot. We will guide you through the process of creating and configuring systemd unit files, ensuring that your partitions are mounted correctly and automatically, regardless of the complexity of your storage setup. By the end of this tutorial, you will have a solid understanding of the benefits of utilizing systemd unit files for partition management, and you will be able to implement these techniques in your own Linux systems.

How to automount file system using systemd unit file in CentOS/RHEL 7 & 8

 

Understanding systemd unit files

A systemd unit file consists of sections, each enclosed within square brackets (e.g., [Unit], [Service], [Mount]). Each section contains key-value pairs, representing configuration options and their respective values. The options and their syntax depend on the type of unit file being defined. Comments in unit files start with a semicolon (;) or a hash symbol (#).

There are several types of unit files, each serving a specific purpose:

  • Service units: Define system services (e.g., web servers, databases) and their behavior. These units have a .service file extension.
  • Mount units: Define filesystem mount points and their properties. These units have a .mount file extension and are the focus of this discussion.
  • Automount units: Configure on-demand mounting of filesystems. These units have a .automount file extension.
  • Device units: Represent kernel devices and expose them to systemd. These units have a .device file extension.
  • Target units: Group other units under a single unit for synchronization and ordering purposes. These units have a .target file extension.
  • Socket units: Define inter-process communication (IPC) sockets for socket-based activation. These units have a .socket file extension.
  • Timer units: Schedule the activation of other units based on time or calendar events. These units have a .timer file extension.
  • Swap units: Manage swap devices or files. These units have a .swap file extension.
  • Path units: Monitor file system changes and activate other units based on these changes. These units have a .path file extension.
  • Slice units: Organize system processes into a hierarchical tree for resource management. These units have a .slice file extension.

Mount units are responsible for managing filesystem mount points in a systemd-managed system. The mount unit file defines the properties of a mount point, such as the partition to be mounted, the target mount point, the filesystem type, and mount options. Systemd automatically generates mount units for entries in /etc/fstab, but you can also create custom mount unit files to gain more control over the mounting process.

The primary sections and options in a mount unit file are:

  • [Unit]: Contains general information about the unit, such as Description and dependency directives like Requires, After, and Before.
  • [Mount]: Specifies the configuration options for the mount point, such as What (device or partition), Where (mount point), Type (filesystem type), and Options (mount options).

 

Steps to mount partition using systemd unit file

Step-1: Identify the partition

Use a tool like lsblk or blkid to identify the partition you want to mount. Note its UUID, device path (e.g., /dev/sdb1), and filesystem type (e.g., ext4).

 

Step-2: Choose the mount point

Determine the desired mount point for the partition (e.g., /mnt/data).

 

Step-3: Create the mount unit file

The mount unit file should be named after the mount point, with slashes replaced by hyphens and the .mount extension added (e.g., mnt-data.mount). Create the file in the /etc/systemd/system/ directory using your preferred text editor:

[root@rhel-8 system]# pwd
/usr/lib/systemd/system

For the sake of this article I will again show the output snippet of my tmp_dir.mount which I created in my older article

[root@rhel-8 system]# cat tmp_dir.mount
#  This file is part of systemd.

[Unit]
Description=Test Directory (/tmp_dir)
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target umount.target
After=swap.target

[Mount]
What=/dev/disk/by-uuid/cea0757d-6329-4bf8-abbf-03f9c313b07f
Where=/tmp_dir
Type=ext4
Options=defaults

[Install]
WantedBy=multi-user.target

Here,

  • Description: A brief description of the mount unit.
  • Requires: (Optional) List other units that must be started before this mount unit.
  • After and Before: (Optional) Specify the order in which the units should be started or stopped.
  • What: The device or partition to be mounted, preferably using the UUID (e.g., UUID=1234-5678-9ABC).
  • Where: The target mount point (e.g., /mnt/data).
  • Type: The filesystem type (e.g., ext4).
  • Options: Mount options, separated by commas (e.g., defaults,noatime).

 

Step-4: Create the automount unit file

Automount units must be named after the automount directories they control. Example: the automount point /home/lennart must be configured in a unit file home-lennart.automount. Here my mount point is under root directory hence my automount systemd unit file will be tmp_dir.automount.

To enable automounting, create a corresponding automount unit file with the .automount extension. Below is my sample automount systemd unit file to automount file system

[root@rhel-8 system]# cat tmp_dir.automount
[Unit]
Description=Sample automount partition
ConditionPathExists=/tmp_dir

[Automount]
Where=/tmp_dir
TimeoutIdleSec=10

[Install]
WantedBy=multi-user.target

Here,

  • Where: Takes an absolute path of a directory of the automount point. If the automount point does not exist at time that the automount point is installed, it is created. This string must be reflected in the unit filename. This option is mandatory.
  • DirectoryMode: Directories of automount points (and any parent directories) are automatically created if needed. This option specifies the file system access mode used when creating these directories. Takes an access mode in octal notation. Defaults to 0755.
  • TimeoutIdleSec: Configures an idleness timeout. Once the mount has been idle for the specified time, systemd will attempt to unmount. Takes a unit-less value in seconds, or a time span value such as "5min 20s". Pass 0 to disable the timeout logic. The timeout is disabled by default.
  • With ConditionPathExists: a file existence condition is checked before a unit is started. If the specified absolute path name does not exist, the condition will fail. If the absolute path name passed to ConditionPathExists= is prefixed with an exclamation mark ("!"), the test is negated, and the unit is only started if the path does not exist.

I have given 10 seconds as the timeout period so that I don't have to wait longer to demonstrate you the behaviour.

Once you have created your automount systemd unit file to automount file system, reload the systemd daemon

[root@rhel-8 system]# systemctl daemon-reload

Make sure /tmp_dir is not in mounted state or else tmp_dir.automount service will fail to start the service to automount file system.

 

Dependency with /etc/fstab

When using systemd, the /etc/fstab file still plays an essential role in managing dependencies and mounting partitions. During the boot process, systemd reads /etc/fstab and automatically generates transient mount and automount unit files for each entry. This allows systemd to mount the partitions specified in /etc/fstab while honoring the traditional configuration.

However, creating custom mount unit files as described earlier gives you more control and flexibility over the mounting process, especially when it comes to handling dependencies and custom options.

When you have a combination of /etc/fstab entries and custom mount unit files, systemd prioritizes the custom unit files. If there is a conflict between a custom mount unit file and an /etc/fstab entry, the custom unit file will take precedence.

To manage dependencies between mount units generated from /etc/fstab and other services, you can use the x-systemd.requires, x-systemd.after, and x-systemd.before mount options in the /etc/fstab file:

  1. x-systemd.requires: Similar to the Requires directive in a custom mount unit file, it specifies that the listed units must be started before this mount point.
  2. x-systemd.after: Similar to the After directive, it indicates that this mount point should be mounted only after the listed units have started successfully.
  3. x-systemd.before: Similar to the Before directive, it specifies that this mount point should be mounted before the listed units.

 

Step 5: Start systemd service

Now let us start tmp_dir.automount service

[root@rhel-8 system]# systemctl start tmp_dir.automount

Let us check the service status. As it shows the service is running but is in (waiting) state since we had given a Time Out value of 10 seconds for idle session

[root@rhel-8 system]# systemctl status tmp_dir.automount
● tmp_dir.automount - Sample automount partition
   Loaded: loaded (/usr/lib/systemd/system/tmp_dir.automount; disabled; vendor preset: disabled)
   Active: active (waiting) since Mon 2019-09-16 18:45:59 IST; 6s ago
    Where: /tmp_dir

Sep 16 18:45:59 rhel-8.example systemd[1]: Set up automount Sample automount partition.
NOTE:
df or mount command will still not show /tmp_dir as mounted yet because automount will mount this filesystem only once someone attempts to access the mount point which is the beauty of this service.

As you see df command does not shows /tmp_dir partition yet

[root@rhel-8 ~]# df -h
Filesystem             Size  Used Avail Use% Mounted on
devtmpfs               900M     0  900M   0% /dev
tmpfs                  915M     0  915M   0% /dev/shm
tmpfs                  915M  8.5M  907M   1% /run
tmpfs                  915M     0  915M   0% /sys/fs/cgroup
/dev/mapper/rhel-root   15G  2.1G   12G  16% /
/dev/sdc1              976M  2.6M  907M   1% /second_part
/dev/sda1              483M  258M  225M  54% /boot
tmpfs                  183M     0  183M   0% /run/user/0

Now if I access this partition using df command explicitly

[root@rhel-8 ~]# df -h /tmp_dir/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1       976M  2.6M  907M   1% /tmp_dir

then the partition will get mounted automatically.

Also if you check the service status, the output snippet will give you a hint of what was happening in the backend

[root@rhel-8 ~]# systemctl status tmp_dir.automount
● tmp_dir.automount - Sample automount partition
   Loaded: loaded (/usr/lib/systemd/system/tmp_dir.automount; disabled; vendor preset: disabled)
   Active: active (waiting) since Mon 2019-09-16 18:45:59 IST; 17min ago
    Where: /tmp_dir

Sep 16 18:45:59 rhel-8.example systemd[1]: Set up automount Sample automount partition.
Sep 16 19:00:54 rhel-8.example systemd[1]: tmp_dir.automount: Got automount request for /tmp_dir, triggered by 3868 (df)
Sep 16 19:01:09 rhel-8.example systemd[1]: tmp_dir.automount: Got automount request for /tmp_dir, triggered by 1318 (bash)
Sep 16 19:02:04 rhel-8.example systemd[1]: tmp_dir.automount: Got automount request for /tmp_dir, triggered by 3903 (df)

 

Summary

This article provides a comprehensive guide on mounting partitions using systemd unit files, a powerful and flexible approach for managing storage devices in modern Linux systems. By leveraging systemd's capabilities, users can enjoy improved dependency handling, custom configuration options, and seamless integration with other systemd services.

The tutorial begins with identifying the partition and its UUID, device path, and filesystem type, followed by choosing a suitable mount point. Next, readers learn to create and configure a mount unit file, including specifying the device, mount point, filesystem type, and mount options. The article also explains how to create an optional automount unit file for on-demand mounting.

Furthermore, the guide delves into handling dependencies between mount units and other services using directives like Requires, After, and Before. These directives help ensure proper startup and shutdown order for the units. The tutorial also touches on the relationship between systemd mount units and traditional /etc/fstab entries, illustrating how systemd can accommodate both methods while prioritizing custom unit files when conflicts arise.

By following this detailed guide, readers will be well-equipped to mount partitions using systemd unit files, enhancing the efficiency and reliability of their Linux systems' storage management.

 

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