systemd

From the project web page:

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. systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, maintains mount and automount points, and implements an elaborate transactional dependency-based service control logic. systemd supports SysV and LSB init scripts and works as a replacement for sysvinit. Other parts include a logging daemon, utilities to control basic system configuration like the hostname, date, locale, maintain a list of logged-in users and running containers and virtual machines, system accounts, runtime directories and settings, and daemons to manage simple network configuration, network time synchronization, log forwarding, and name resolution.

Note: For a detailed explanation of why Arch moved to systemd, see this forum post.

Basic systemctl usage

The main command used to introspect and control systemd is systemctl. Some of its uses are examining the system state and managing the system and services. See systemctl(1) for more details.

Tip:

You can use all of the following systemctl commands with the -H user@host switch to control a systemd instance on a remote machine. This will use SSH to connect to the remote systemd instance.
Plasma users can install systemd-kcm^AUR as a graphical frontend for systemctl. After installing, the module will be added under System administration.

Using units

Units commonly include, but are not limited to, services (.service), mount points (.mount), devices (.device) and sockets (.socket).

When using systemctl, you generally have to specify the complete name of the unit file, including its suffix, for example sshd.socket. There are however a few short forms when specifying the unit in the following systemctl commands:

If you do not specify the suffix, systemctl will assume .service. For example, netctl and netctl.service are equivalent.
Mount points will automatically be translated into the appropriate .mount unit. For example, specifying /home is equivalent to home.mount.
Similar to mount points, devices are automatically translated into the appropriate .device unit, therefore specifying /dev/sda2 is equivalent to dev-sda2.device.

See systemd.unit(5) for details.

Note: Some unit names contain an @ sign (e.g. name@string.service): this means that they are instances of a template unit, whose actual file name does not contain the string part (e.g. [email protected]). string is called the instance identifier, and is similar to an argument that is passed to the template unit when called with the systemctl command: in the unit file it will substitute the %i specifier. To be more accurate, before trying to instantiate the name@.suffix template unit, systemd will actually look for a unit with the exact name@string.suffix file name, although by convention such a "clash" happens rarely, i.e. most unit files containing an @ sign are meant to be templates. Also, if a template unit is called without an instance identifier, it will generally fail (except with certain systemctl commands, like cat).

The commands in the below table operate on system units since --system is the implied default for systemctl. To instead operate on user units (for the calling user), use systemctl --user without root privileges. See also systemd/User#Basic setup to enable/disable user units for all users.

Tip:

Most commands also work if multiple units are specified, see systemctl(1) for more information.
The --now switch can be used in conjunction with enable, disable, and mask to respectively start, stop, or mask the unit immediately rather than after rebooting.
A package may offer units for different purposes. If you just installed a package, pacman -Qql package | grep -Fe .service -e .socket can be used to check and find them.

Action	Command	Note
Analyzing the system state
Show system status	`systemctl status`
List running units	`systemctl` or `systemctl list-units`
List failed units	`systemctl --failed`
List installed unit files¹	`systemctl list-unit-files`
Show process status for a PID	`systemctl status pid`	cgroup slice, memory and parent
Checking the unit status
Show a manual page associated with a unit	`systemctl help unit`	as supported by the unit
Status of a unit	`systemctl status unit`	including whether it is running or not
Check whether a unit is enabled	`systemctl is-enabled unit`
Starting, restarting, reloading a unit
Start a unit immediately	`systemctl start unit` as root
Stop a unit immediately	`systemctl stop unit` as root
Restart a unit	`systemctl restart unit` as root
Reload a unit and its configuration	`systemctl reload unit` as root
Reload systemd manager configuration²	`systemctl daemon-reload` as root	scan for new or changed units
Enabling a unit
Enable a unit to start automatically at boot	`systemctl enable unit` as root
Enable a unit to start automatically at boot and start it immediately	`systemctl enable --now unit` as root
Disable a unit to no longer start at boot	`systemctl disable unit` as root
Reenable a unit³	`systemctl reenable unit` as root	i.e. disable and enable anew
Masking a unit
Mask a unit to make it impossible to start⁴	`systemctl mask unit` as root
Unmask a unit	`systemctl unmask unit` as root

See systemd.unit(5) § UNIT FILE LOAD PATH for the directories where available unit files can be found.
This does not ask the changed units to reload their own configurations (see the Reload action).
For example, in case its [Install] section has changed since last enabling it.
Both manually and as a dependency, which makes masking dangerous. Check for existing masked units with:
```
# systemctl list-unit-files --state=masked
```

Power management

polkit is necessary for power management as an unprivileged user. If you are in a local systemd-logind user session and no other session is active, the following commands will work without root privileges. If not (for example, because another user is logged into a tty), systemd will automatically ask you for the root password.

Action	Command
Shut down and reboot the system	`systemctl reboot`
Shut down and power-off the system	`systemctl poweroff`
Suspend the system	`systemctl suspend`
Put the system into hibernation	`systemctl hibernate`
Put the system into hybrid-sleep state (or suspend-to-both)	`systemctl hybrid-sleep`

Writing unit files

The syntax of systemd's unit files (systemd.unit(5)) is inspired by XDG Desktop Entry Specification .desktop files, which are in turn inspired by Microsoft Windows .ini files. Unit files are loaded from multiple locations (to see the full list, run systemctl show --property=UnitPath), but the main ones are (listed from lowest to highest precedence):

/usr/lib/systemd/system/: units provided by installed packages
/etc/systemd/system/: units installed by the system administrator

Note:

The load paths are completely different when running systemd in user mode.
systemd unit names may only contain ASCII alphanumeric characters, underscores and periods. All other characters must be replaced by C-style "\x2d" escapes, or employ their predefined semantics ('@', '-'). See systemd.unit(5) and systemd-escape(1) for more information.

Look at the units installed by your packages for examples, as well as systemd.service(5) § EXAMPLES.

Tip: Comments prepended with # may be used in unit-files as well, but only in new lines. Do not use end-line comments after systemd parameters or the unit will fail to activate.

Handling dependencies

With systemd, dependencies can be resolved by designing the unit files correctly. The most typical case is that the unit A requires the unit B to be running before A is started. In that case add Requires=B and After=B to the [Unit] section of A. If the dependency is optional, add Wants=B and After=B instead. Note that Wants= and Requires= do not imply After=, meaning that if After= is not specified, the two units will be started in parallel.

Dependencies are typically placed on services and not on #Targets. For example, network.target is pulled in by whatever service configures your network interfaces, therefore ordering your custom unit after it is sufficient since network.target is started anyway.

Service types

There are several different start-up types to consider when writing a custom service file. This is set with the Type= parameter in the [Service] section:

Type=simple (default): systemd considers the service to be started up immediately. The process must not fork. Do not use this type if other services need to be ordered on this service, unless it is socket activated.
Type=forking: systemd considers the service started up once the process forks and the parent has exited. For classic daemons use this type unless you know that it is not necessary. You should specify PIDFile= as well so systemd can keep track of the main process.
Type=oneshot: this is useful for scripts that do a single job and then exit. You may want to set RemainAfterExit=yes as well so that systemd still considers the service as active after the process has exited.
Type=notify: identical to Type=simple, but with the stipulation that the daemon will send a signal to systemd when it is ready. The reference implementation for this notification is provided by libsystemd-daemon.so.
Type=dbus: the service is considered ready when the specified BusName appears on DBus's system bus.
Type=idle: systemd will delay execution of the service binary until all jobs are dispatched. Other than that behavior is very similar to Type=simple.

See the systemd.service(5) § OPTIONS man page for a more detailed explanation of the Type values.

Editing provided units

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

Reason: Should be renamed to more descriptive Modifying provided units. (Discuss in Talk:Edit#Deprecation)

To avoid conflicts with pacman, unit files provided by packages should not be directly edited. There are two safe ways to modify a unit without touching the original file: create a new unit file which overrides the original unit or create drop-in snippets which are applied on top of the original unit. For both methods, you must reload the unit afterwards to apply your changes. This can be done either by editing the unit with systemctl edit (which reloads the unit automatically) or by reloading all units with:

# systemctl daemon-reload

Tip:

You can use systemd-delta to see which unit files have been overridden or extended and what exactly has been changed.
Use systemctl cat unit to view the content of a unit file and all associated drop-in snippets.

Replacement unit files

To replace the unit file /usr/lib/systemd/system/unit, create the file /etc/systemd/system/unit and reenable the unit to update the symlinks.

Alternatively, run:

# systemctl edit --full unit

This opens /etc/systemd/system/unit in your editor (copying the installed version if it does not exist yet) and automatically reloads it when you finish editing.

Note: The replacement units will keep on being used even if Pacman updates the original units in the future. This method makes system maintenance more difficult and therefore the next approach is preferred.

Drop-in files

To create drop-in files for the unit file /usr/lib/systemd/system/unit, create the directory /etc/systemd/system/unit.d/ and place .conf files there to override or add new options. systemd will parse and apply these files on top of the original unit.

The easiest way to do this is to run:

# systemctl edit unit

This opens the file /etc/systemd/system/unit.d/override.conf in your text editor (creating it if necessary) and automatically reloads the unit when you are done editing.

Note: Not all keys can be overridden with drop-in files. For example, for changing Conflicts= a replacement file is necessary.

Revert to vendor version

To revert any changes to a unit made using systemctl edit do:

# systemctl revert unit

Examples

For example, if you simply want to add an additional dependency to a unit, you may create the following file:

/etc/systemd/system/unit.d/customdependency.conf

[Unit]
Requires=new dependency
After=new dependency

As another example, in order to replace the ExecStart directive, create the following file:

/etc/systemd/system/unit.d/customexec.conf

[Service]
ExecStart=
ExecStart=new command

Note how ExecStart must be cleared before being re-assigned [1]. The same holds for every item that can be specified multiple times, e.g. OnCalendar for timers.

One more example to automatically restart a service:

/etc/systemd/system/unit.d/restart.conf

[Service]
Restart=always
RestartSec=30

Targets

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

Reason: Unclear description, copy-pasted content (explicitly mentions "Fedora"). (Discuss in Talk:Systemd#Make section "Targets" more clearly)

systemd uses targets to group units together via dependencies and as standardized synchronization points. They serve a similar purpose as runlevels but act a little differently. Each target is named instead of numbered and is intended to serve a specific purpose with the possibility of having multiple ones active at the same time. Some targets are implemented by inheriting all of the services of another target and adding additional services to it. There are systemd targets that mimic the common SystemVinit runlevels so you can still switch targets using the familiar telinit RUNLEVEL command.

Get current targets

The following should be used under systemd instead of running runlevel:

$ systemctl list-units --type=target

Create custom target

The runlevels that held a defined meaning under sysvinit (i.e., 0, 1, 3, 5, and 6); have a 1:1 mapping with a specific systemd target. Unfortunately, there is no good way to do the same for the user-defined runlevels like 2 and 4. If you make use of those it is suggested that you make a new named systemd target as /etc/systemd/system/your target that takes one of the existing runlevels as a base (you can look at /usr/lib/systemd/system/graphical.target as an example), make a directory /etc/systemd/system/your target.wants, and then symlink the additional services from /usr/lib/systemd/system/ that you wish to enable.

Mapping between SysV runlevels and systemd targets

SysV Runlevel	systemd Target	Notes
0	runlevel0.target, poweroff.target	Halt the system.
1, s, single	runlevel1.target, rescue.target	Single user mode.
2, 4	runlevel2.target, runlevel4.target, multi-user.target	User-defined/Site-specific runlevels. By default, identical to 3.
3	runlevel3.target, multi-user.target	Multi-user, non-graphical. Users can usually login via multiple consoles or via the network.
5	runlevel5.target, graphical.target	Multi-user, graphical. Usually has all the services of runlevel 3 plus a graphical login.
6	runlevel6.target, reboot.target	Reboot
emergency	emergency.target	Emergency shell

Change current target

In systemd, targets are exposed via target units. You can change them like this:

# systemctl isolate graphical.target

This will only change the current target, and has no effect on the next boot. This is equivalent to commands such as telinit 3 or telinit 5 in Sysvinit.

Change default target to boot into

The standard target is default.target, which is a symlink to graphical.target. This roughly corresponds to the old runlevel 5.

To verify the current target with systemctl:

$ systemctl get-default

To change the default target to boot into, change the default.target symlink. With systemctl:

# systemctl set-default multi-user.target

Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target -> /usr/lib/systemd/system/multi-user.target.

Alternatively, append one of the following kernel parameters to your bootloader:

systemd.unit=multi-user.target (which roughly corresponds to the old runlevel 3),
systemd.unit=rescue.target (which roughly corresponds to the old runlevel 1).

Default target order

systemd chooses the default.target according to the following order:

Kernel parameter shown above
Symlink of /etc/systemd/system/default.target
Symlink of /usr/lib/systemd/system/default.target

systemd components

Some (not exhaustive) components of systemd are:

systemd-boot — simple UEFI boot manager;
systemd-firstboot — basic system setting initialization before first boot;
systemd-homed — portable human-user accounts;
systemd-logind(8) — session management;
systemd-networkd — network configuration management;
systemd-nspawn — light-weight namespace container;
systemd-resolved — network name resolution;
systemd-sysusers(8) — creates system users and groups and adds users to groups at package installation or boot time;
systemd-timesyncd — system time synchronization across the network;
systemd/Journal — system logging;
systemd/Timers — monotonic or realtime timers for controlling .service files or events, reasonable alternative to cron.
systemd-stub(7) — a UEFI boot stub used for creating unified kernel images.

systemd.mount - mounting

systemd is in charge of mounting the partitions and filesystems specified in /etc/fstab. The systemd-fstab-generator(8) translates all the entries in /etc/fstab into systemd units; this is performed at boot time and whenever the configuration of the system manager is reloaded.

systemd extends the usual fstab capabilities and offers additional mount options. These affect the dependencies of the mount unit. They can, for example, ensure that a mount is performed only once the network is up or only once another partition is mounted. The full list of specific systemd mount options, typically prefixed with x-systemd., is detailed in systemd.mount(5) § FSTAB.

An example of these mount options is automounting, which means mounting only when the resource is required rather than automatically at boot time. This is provided in fstab#Automount with systemd.

GPT partition automounting

On UEFI-booted systems, if specific conditions are met, systemd-gpt-auto-generator(8) will automount GPT partitions following the Discoverable Partitions Specification and they can thus be omitted from fstab.

The prerequisites are:

The boot loader must set the LoaderDevicePartUUID EFI variable, so that the used EFI system partition can be identified. This is supported by systemd-boot, systemd-stub(7) and rEFInd (not enabled by default). This can be verified by running bootctl and checking the status of Boot loader sets ESP information.
The root partition must be on the same physical disk as the used EFI system partition. Other partitions that will be automounted must be on the same physical disk as the root partition. This basically means that all automounted partitions must share the same physical disk with the ESP.

Tip: The automounting of a partition can be disabled by changing the partition's type GUID or setting the partition attribute bit 63 "do not automount", see gdisk#Prevent GPT partition automounting.

/var

For /var automounting to work, the PARTUUID must match the SHA256 HMAC hash of the partition type UUID (4d21b016-b534-45c2-a9fb-5c16e091fd2d) keyed by the machine ID. The required PARTUUID can be obtained using:

$ systemd-id128 -u --app-specific=4d21b016-b534-45c2-a9fb-5c16e091fd2d machine-id

Note: systemd-id128(1) reads the machine ID from /etc/machine-id, this makes it impossible to know the needed PARTUUID before the system is installed.

systemd-sysvcompat

The primary role of systemd-sysvcompat (required by base) is to provide the traditional linux init binary. For systemd-controlled systems, init is just a symbolic link to its systemd executable.

In addition, it provides four convenience shortcuts that SysVinit users might be used to. The convenience shortcuts are halt(8), poweroff(8), reboot(8) and shutdown(8). Each one of those four commands is a symbolic link to systemctl, and is governed by systemd behavior. Therefore, the discussion at #Power management applies.

systemd-based systems can give up those System V compatibility methods by using the init= boot parameter (see, for example, /bin/init is in systemd-sysvcompat ?) and systemd native systemctl command arguments.

systemd-tmpfiles - temporary files

systemd-tmpfiles creates, deletes and cleans up volatile and temporary files and directories. It reads configuration files in /etc/tmpfiles.d/ and /usr/lib/tmpfiles.d/ to discover which actions to perform. Configuration files in the former directory take precedence over those in the latter directory.

Configuration files are usually provided together with service files, and they are named in the style of /usr/lib/tmpfiles.d/program.conf. For example, the Samba daemon expects the directory /run/samba to exist and to have the correct permissions. Therefore, the samba package ships with this configuration:

/usr/lib/tmpfiles.d/samba.conf

D /run/samba 0755 root root

Configuration files may also be used to write values into certain files on boot. For example, if you used /etc/rc.local to disable wakeup from USB devices with echo USBE > /proc/acpi/wakeup, you may use the following tmpfile instead:

/etc/tmpfiles.d/disable-usb-wake.conf

#    Path                  Mode UID  GID  Age Argument
w    /proc/acpi/wakeup     -    -    -    -   USBE

See the systemd-tmpfiles(8) and tmpfiles.d(5) man pages for details.

Note: This method may not work to set options in /sys since the systemd-tmpfiles-setup service may run before the appropriate device modules are loaded. In this case, you could check whether the module has a parameter for the option you want to set with modinfo module and set this option with a config file in /etc/modprobe.d. Otherwise, you will have to write a udev rule to set the appropriate attribute as soon as the device appears.

Tips and tricks

GUI configuration tools

systemadm — Graphical browser for systemd units. It can show the list of units, possibly filtered by type.

https://cgit.freedesktop.org/systemd/systemd-ui/ || systemd-ui

SystemdGenie — systemd management utility based on KDE technologies.

https://invent.kde.org/system/systemdgenie || systemdgenie

Running services after the network is up

To delay a service until after the network is up, include the following dependencies in the .service file:

/etc/systemd/system/foo.service

[Unit]
...
Wants=network-online.target
After=network-online.target
...

The network wait service of the particular application that manages the network, must also be enabled so that network-online.target properly reflects the network status.

If using NetworkManager, NetworkManager-wait-online.service is enabled together with NetworkManager.service. Check if this is the case with systemctl is-enabled NetworkManager-wait-online.service. If it is not enabled, then reenable NetworkManager.service.
In the case of netctl, enable the netctl-wait-online.service.
If using systemd-networkd, systemd-networkd-wait-online.service is enabled together with systemd-networkd.service. Check if this is the case with systemctl is-enabled systemd-networkd-wait-online.service. If it is not enabled, then reenable systemd-networkd.service.

For more detailed explanations see the discussion in the Network configuration synchronization points.

If a service needs to perform DNS queries, it should additionally be ordered after nss-lookup.target:

/etc/systemd/system/foo.service

[Unit]
...
Wants=network-online.target
After=network-online.target nss-lookup.target
...

See systemd.special(7) § Special Passive System Units.

For nss-lookup.target to have any effect it needs a service that pulls it in via Wants=nss-lookup.target and orders itself before it with Before=nss-lookup.target. Typically this is done by local DNS resolvers.

Check which active service, if any, is pulling in nss-lookup.target with:

$ systemctl list-dependencies --reverse nss-lookup.target

Enable installed units by default

This article or section needs expansion.

Reason: How does it work with instantiated units? (Discuss in Talk:Systemd)

Arch Linux ships with /usr/lib/systemd/system-preset/99-default.preset containing disable *. This causes systemctl preset to disable all units by default, such that when a new package is installed, the user must manually enable the unit.

If this behavior is not desired, simply create a symlink from /etc/systemd/system-preset/99-default.preset to /dev/null in order to override the configuration file. This will cause systemctl preset to enable all units that get installed—regardless of unit type—unless specified in another file in one systemctl preset's configuration directories. User units are not affected. See systemd.preset(5) for more information.

Note: Enabling all units by default may cause problems with packages that contain two or more mutually exclusive units. systemctl preset is designed to be used by distributions and spins or system administrators. In the case where two conflicting units would be enabled, you should explicitly specify which one is to be disabled in a preset configuration file as specified in the systemd.preset(5) man page.

Sandboxing application environments

This article or section is a candidate for moving to systemd/Sandboxing.

Notes: The topic is broad enough for a dedicated page. See User:NetSysFire/systemd sandboxing for a proposed draft. (Discuss in Talk:Security#systemd unit hardening and system.conf tweaks)

A unit file can be created as a sandbox to isolate applications and their processes within a hardened virtual environment. systemd leverages namespaces, a list of allowed/denied capabilities, and control groups to container processes through an extensive execution environment configuration—systemd.exec(5).

The enhancement of an existing systemd unit file with application sandboxing typically requires trial-and-error tests accompanied by the generous use of strace, stderr and journalctl(1) error logging and output facilities. You may want to first search upstream documentation for already done tests to base trials on. To get a starting point for possible hardening options, run

$ systemd-analyze security unit

Some examples of how sandboxing with systemd can be deployed:

CapabilityBoundingSet defines a list of capabilities(7) that are allowed or denied for a unit. See systemd.exec(5) § CAPABILITIES.
- The CAP_SYS_ADM capability, for example, which should be one of the goals of a secure sandbox: CapabilityBoundingSet=~ CAP_SYS_ADM

Notifying about failed services

In order to notify about service failures, a OnFailure= directive needs to be added to the according service file, for example by using a drop-in configuration file. Adding this directive to every service unit can be achieved with a top-level drop-in configuration file. For details about top-level drop-ins, see systemd.unit(5).

Create a top-level drop-in for services:

/etc/systemd/system/service.d/toplevel-override.conf

[Unit]
OnFailure=failure-notification@%n

This adds OnFailure=failure-notification@%n to every service file. If some_service_unit fails, failure-notification@some_service_unit will be started to handle the notification delivery (or whatever task it is configured to perform).

Create the failure-notification@ template unit:

/etc/systemd/system/[email protected]

[Unit]
Description=Send a notification about a failed systemd unit
After=network.target

[Service]
Type=simple
ExecStart=/path/to/failure-notification.sh %i

You can create the failure-notification.sh script and define what to do or how to notify (mail, gotify, xmpp, etc.). The %i will be the name of the failed service unit and will be passed as argument to the script.

In order to prevent a recursion for starting instances of [email protected] again and again if the start fails, create an empty drop-in configuration file with the same name as the top-level drop-in (the empty service-level drop-in configuration file takes precedence over the top-level drop-in and overrides the latter one):

# mkdir -p /etc/systemd/system/[email protected]
# touch /etc/systemd/system/[email protected]/toplevel-override.conf

Troubleshooting

Investigating failed services

To find the systemd services which failed to start:

$ systemctl --state=failed

To find out why they failed, examine their log output. See systemd/Journal#Filtering output for details.

Diagnosing boot problems

systemd has several options for diagnosing problems with the boot process. See boot debugging for more general instructions and options to capture boot messages before systemd takes over the boot process. Also see freedesktop.org's systemd debugging documentation.

Diagnosing a service

If some systemd service misbehaves or you want to get more information about what is happening, set the SYSTEMD_LOG_LEVEL environment variable to debug. For example, to run the systemd-networkd daemon in debug mode:

Add a drop-in file for the service adding the two lines:

[Service]
Environment=SYSTEMD_LOG_LEVEL=debug

Or equivalently, set the environment variable manually:

# SYSTEMD_LOG_LEVEL=debug /lib/systemd/systemd-networkd

then restart systemd-networkd and watch the journal for the service with the -f/--follow option.

Shutdown/reboot takes terribly long

If the shutdown process takes a very long time (or seems to freeze) most likely a service not exiting is to blame. systemd waits some time for each service to exit before trying to kill it. To find out if you are affected, see Shutdown completes eventually in the systemd wiki.

A common problem is a stalled shutdown or suspend process. To verify if it is the case you could run either of these commands and check the outputs

# systemctl poweroff

Failed to power off system via logind: There's already a shutdown or sleep operation in progress

# systemctl list-jobs

JOB UNIT                    TYPE  STATE  
...
21593 systemd-suspend.service start running
21592 suspend.target          start waiting
..

The solution to this would be to cancel these jobs by running

# systemctl cancel
# systemctl stop systemd-suspend.service

and then trying shutdown or reboot again.

Short lived processes do not seem to log any output

If running journalctl -u foounit as root does not show any output for a short lived service, look at the PID instead. For example, if systemd-modules-load.service fails, and systemctl status systemd-modules-load shows that it ran as PID 123, then you might be able to see output in the journal for that PID, i.e. by running journalctl -b _PID=123 as root. Metadata fields for the journal such as _SYSTEMD_UNIT and _COMM are collected asynchronously and rely on the /proc directory for the process existing. Fixing this requires fixing the kernel to provide this data via a socket connection, similar to SCM_CREDENTIALS. In short, it is a bug. Keep in mind that immediately failed services might not print anything to the journal as per design of systemd.

Boot time increasing over time

The factual accuracy of this article or section is disputed.

Reason: NetworkManager issues are not systemd's fault, the alleged reports are missing. Slow systemctl status or journalctl do not affect boot time. (Discuss in Talk:Systemd)

After using systemd-analyze a number of users have noticed that their boot time has increased significantly in comparison with what it used to be. After using systemd-analyze blame NetworkManager is being reported as taking an unusually large amount of time to start.

The problem for some users has been due to /var/log/journal becoming too large. This may have other impacts on performance, such as for systemctl status or journalctl. As such the solution is to remove every file within the folder (ideally making a backup of it somewhere, at least temporarily) and then setting a journal file size limit as described in Systemd/Journal#Journal size limit.

systemd-tmpfiles-setup.service fails to start at boot

Starting with systemd 219, /usr/lib/tmpfiles.d/systemd.conf specifies ACL attributes for directories under /var/log/journal and, therefore, requires ACL support to be enabled for the filesystem the journal resides on.

See Access Control Lists#Enable ACL for instructions on how to enable ACL on the filesystem that houses /var/log/journal.

Disable emergency mode on remote machine

You may want to disable emergency mode on a remote machine, for example, a virtual machine hosted at Azure or Google Cloud. It is because if emergency mode is triggered, the machine will be blocked from connecting to network.

To disable it, mask emergency.service and emergency.target.