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 service units. The systemd_service_units_state alert indicates that one or more of the systemd service units are in the failed state. One of the following reasons can cause this alert:

  • The process of the service returns an error code on exit.
  • The process of the service crashed.
  • An operation timed out occurred.
  • The service failed after too many restarts.
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-services

A unit configuration file whose name ends in .service encodes information about a process controlled and supervised by systemd. To view, start, stop, restart, enable, or disable system services, use the systemctl command line interface. It is common that services are ordered to start after some specified service that depends on (try the command systemctl list-dependencies --before|after <service_name>.service)

See more in the man pages, man systemd.service

References and source
  1. systemd on wikipedia

Troubleshooting section:

General approach

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

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

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

    root@netdata~ # journalctl -u <service_name>.service -f 

    This command will monitor the journalctl log messages for your service.

  3. In a new terminal, try to restart the service.

    root@netdata~ # systemctl restart <service_name>.service 

    This command will restart your service.

  4. Check the log messages from the command of step 2.

Run the service in debug mode
  1. Identify which service units fail. Open the Netdata dashboard, find the current active alarms under the active alarms tab and look into its chart. (systemdunits_service_units.service_unit_state). In this chart, identify which service units are in state with value 5

  2. Stop the service

    root@netdata~ # systemctl stop <service_name>.service 
  3. Try to start it with the SYSTEMD_LOG_LEVEL=debug env variable. Let’s assume in our case we want to debug the systemd-networkd service.

    root@netdata~ # SYSTEMD_LOG_LEVEL=debug /lib/systemd/systemd-networkd

    This command will start your service in debug mode.

  4. Check the log messages.