Using systemd drop-in units
There are two methods of overriding default Flatcar Container Linux settings in unit files: copying the unit file from /usr/lib64/systemd/system
to /etc/systemd/system
and modifying the chosen settings. Alternatively, one can create a directory named unit.d
within /etc/systemd/system
and place a drop-in file name.conf
there that only changes the specific settings one is interested in. Note that multiple such drop-in files are read if present.
The advantage of the first method is that one easily overrides the complete unit, the default Flatcar Container Linux unit is not parsed at all anymore. It has the disadvantage that improvements to the unit file supplied by Flatcar Container Linux are not automatically incorporated on updates.
The advantage of the second method is that one only overrides the settings one specifically wants, where updates to the original Flatcar Container Linux unit automatically apply. This has the disadvantage that some future Flatcar Container Linux updates might be incompatible with the local changes, but the risk is much lower.
Note that for drop-in files, if one wants to remove entries from a setting that is parsed as a list (and is not a dependency), such as ConditionPathExists=
(or e.g. ExecStart=
in service units), one needs to first clear the list before re-adding all entries except the one that is to be removed. See below for an example.
This also applies for user instances of systemd, but with different locations for the unit files. See the section on unit load paths in official systemd doc for further details.
Example: customizing locksmithd.service
Let’s review /usr/lib64/systemd/system/locksmithd.service
unit (you can find it using this command: systemctl list-units | grep locksmithd
) with the following contents:
[Unit]
Description=Cluster reboot manager
After=update-engine.service
ConditionVirtualization=!container
ConditionPathExists=!/usr/.noupdate
[Service]
CPUShares=16
MemoryLimit=32M
PrivateDevices=true
Environment=GOMAXPROCS=1
EnvironmentFile=-/usr/share/flatcar/update.conf
EnvironmentFile=-/etc/flatcar/update.conf
ExecStart=/usr/lib/locksmith/locksmithd
Restart=on-failure
RestartSec=10s
[Install]
WantedBy=multi-user.target
Let’s walk through increasing the RestartSec
parameter via both methods:
Override only specific option
You can create a drop-in file /etc/systemd/system/locksmithd.service.d/10-restart_60s.conf
with the following contents:
[Service]
RestartSec=60s
Then reload systemd, scanning for new or changed units:
systemctl daemon-reload
And restart modified service if necessary (in our example we have changed only RestartSec
option, but if you want to change environment variables, ExecStart
or other run options you have to restart service):
systemctl restart locksmithd.service
Here is how that could be implemented within a Butane Config:
variant: flatcar
version: 1.0.0
systemd:
units:
- name: locksmithd.service
enabled: true
dropins:
- name: 10-restart_60s.conf
contents: |
[Service]
RestartSec=60s
This change is small and targeted. It is the easiest way to tweak unit’s parameters.
Override the whole unit file
Another way is to override whole systemd unit. Copy default unit file /usr/lib64/systemd/system/locksmithd.service
to /etc/systemd/system/locksmithd.service
and change the chosen settings:
[Unit]
Description=Cluster reboot manager
After=update-engine.service
ConditionVirtualization=!container
ConditionPathExists=!/usr/.noupdate
[Service]
CPUShares=16
MemoryLimit=32M
PrivateDevices=true
Environment=GOMAXPROCS=1
EnvironmentFile=-/usr/share/flatcar/update.conf
EnvironmentFile=-/etc/flatcar/update.conf
ExecStart=/usr/lib/locksmith/locksmithd
Restart=on-failure
RestartSec=60s
[Install]
WantedBy=multi-user.target
Butane Config example:
variant: flatcar
version: 1.0.0
systemd:
units:
- name: locksmithd.service
enabled: true
contents: |
[Unit]
Description=Cluster reboot manager
After=update-engine.service
ConditionVirtualization=!container
ConditionPathExists=!/usr/.noupdate
[Service]
CPUShares=16
MemoryLimit=32M
PrivateDevices=true
Environment=GOMAXPROCS=1
EnvironmentFile=-/usr/share/flatcar/update.conf
EnvironmentFile=-/etc/flatcar/update.conf
ExecStart=/usr/lib/locksmith/locksmithd
Restart=on-failure
RestartSec=60s
[Install]
WantedBy=multi-user.target
List drop-ins
To see all runtime drop-in changes for system units run the command below:
systemd-delta --type=extended
Other systemd examples
For more examples using systemd customization, check out these documents: