Linux | Systemd units

Systemd is a suite of basic building blocks for a Linux system. It provides a system and service manager that runs as PID 1 and starts the rest of the system.

The Netdata Agent monitors the systemd.mount units state. The systemd_mount_units_state alert indicates that one or more of the systemd.mount units are in the failed state. A systemd mount unit “failed” when the service process returned error code on exit, or crashed, an operation timed out, or after too many restarts. The cause of a failed states is stored in a log

Read more about systemd

Here is some useful information about systemd from wikipedia 1

Systemd includes features like on-demand starting of daemons, snapshot support, process tracking, and Inhibitor Locks. Systemd is not just the name of the init daemon, but also refers to the entire software bundle around it, which, in addition to the systemd init daemon, includes the daemons journald, logind and networkd, and many other low-level components. In January 2013, Poettering described systemd not as one program, but rather a large software suite that includes 69 individual binaries. As an integrated software suite, systemd replaces the startup sequences and runlevels controlled by the traditional init daemon, along with the shell scripts executed under its control. systemd also integrates many other services that are common on Linux systems by handling user logins, the system console, device hotplugging, scheduled execution (replacing cron), logging, hostnames and locales.

Like the init daemon, systemd is a daemon that manages other daemons, which, including systemd itself, are background processes. systemd is the first daemon to start during booting and the last daemon to terminate during shutdown. The systemd daemon serves as the root of the user space’s process tree. The first process (PID1) has a special role on Unix systems, as it replaces the parent of a process when the original parent terminates. Therefore, the first process is particularly well suited for the purpose of monitoring daemons.

Systemd executes elements of its startup sequence in parallel, which is theoretically faster than the traditional startup sequence approach. For inter-process communication (IPC), systemd makes Unix domain sockets and D-Bus available to the running daemons. The state of systemd itself can also be preserved in a snapshot for future recall.

Systemd’s core components include the following:

  • systemd is a system and service manager for Linux operating systems.

  • systemctl is a command to introspect and control the state of the systemd system and service
    manager. Not to be confused with sysctl.

  • systemd-analyze may be used to determine system boot-up performance statistics and retrieve
    other state and tracing information from the system and service manager.

See more on systemd-mount units

A unit configuration file whose name ends in .mount encodes information about a file system mount
point controlled and supervised by systemd. Additional options are listed in systemd.exec(5), which define the execution environment the mount(8) program is executed in, and in systemd.kill(5), which define the way the processes are terminated, and in systemd.resource-control(5), which configure resource control settings for the processes of the service.

The options User= and Group= are not useful for mount units. systemd passes two parameters to mount(8) the values of What= and Where=. When invoked in this way, mount(8) does not read any
options from /etc/fstab, and must be run as UID 0.

Mount units must be named after the mount point directories they control. For instance, the mount point /home/lennart must be configured in a unit file home-lennart.mount. For details about the escaping logic used to convert a file system path to a unit name, see systemd.unit(5). Note that mount units cannot be templated, nor is possible to add multiple names to a mount unit by creating additional symlinks to it.

Mount units may either be configured via unit files, or via /etc/fstab (see man fstab for details). Mounts listed in /etc/fstab will be converted into native units dynamically at boot and when the configuration of the system manager is reloaded. In general, configuring mount points through /etc/fstab is the preferred approach. See systemd-fstab-generator(8) for details about the conversion. 2

References and source
  1. systemd on wikipedia
  2. man page for systemd.mount

Troubleshooting section:

General approach

When a mount is in failed state, you should always try to gather more information about it.

  1. Identify which mount units fail. Open the Netdata dashboard, find the current active alarms under the active alarms tab and look into its chart. (systemdunits_mount_units.mount_unit_state). In this chart, identify which mount units are in state with value 5

  2. Gather more information about the failing mount. We advise you to run the following command in a second terminal.

    root@netdata~ # journalctl -u <mount_name>.mount -f 

    This command will monitor the journalctl log messages for your mount unit.

  3. In your main terminal, try mount the mount manually.

    root@netdata~ # mount -v <mount_name> 

    This command will try to mount your mount unit in verbose mode.

  4. Check the output messages from both terminals for abnormalities.

Verify the fstab configuration
  1. Open a terminal and run the following command

    root@netdata~ # sudo findmnt --verify --verbose

This command will check mount table content (default: /etc/fstab) in verbose mode