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 path units state. This alert indicates that one or more of the systemd path units are in the failed state. A systemd path 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.path units

A unit configuration file whose name ends in “.path” encodes information about a path monitored by system. With path units, you can monitor files and directories for certain events. If a specified event occurs, a service unit is executed, and it usually carries the same name as the path unit.

In the [Path] section, PathChanged= specifies the absolute path to the file to be monitored, while Unit= indicates which service unit to execute if the file changes. 2

Path units are very useful to monitor files for changes with systemd.

References and source
  1. systemd on wikipedia
  2. A Brief introduction to path units by Jörg Kastning

Troubleshooting section:

General approach

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

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

  2. Gather more information about the failing path unit and the service that manages it (in most of the cases they will have the same name). We advise you to run the following commands in two different terminals.

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

    These commands will monitor the journalctl log messages for your path/service unit.

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

    root@netdata~ # systemctl restart <service_name>.service 
  4. Check the log messages from the commands of step 2.

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

  2. Stop the service that manages this path (in most of the cases the service will have the same name
    with the path)

    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
  4. Check the log messages.