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 target units state. Receiving this alert indicates that one or more of the systemd target units are in the failed state. A systemd target 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 units

Target units file ends with the .target file extension and their only purpose is to group together
other systemd units through a chain of dependencies. For example, the unit, which
is used to start a graphical session, starts system services such as the GNOME Display Manager (
gdm.service) or Accounts Service (accounts-daemon .service) and also activates the
unit. Similarly, the unit starts other essential system services such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and activates another target unit named 2

Among other things, target units are a more flexible replacement for SysV runlevels in the classic
SysV init system. For compatibility reasons special target units such as exist which are used by the SysV runlevel compatibility code in systemd.

References and source
  1. systemd on wikipedia
  2. explained on Redhat’s documentantion

Troubleshooting section:

General approach

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

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

  2. Gather more information about the failing target unit

    root@netdata~ # systemctl status <target_name>.target
  3. Check the log messages from the command of step 2.