Problem:
When logging into your CoreOS instance, you see this warning message:
############################################################################ WARNING: This system is using cgroups v1. For increased reliability it is strongly recommended to migrate this system and your workloads to use cgroups v2. For instructions on how to adjust kernel arguments to use cgroups v2, see: https://docs.fedoraproject.org/en-US/fedora-coreos/kernel-args/ To disable this warning, use: sudo systemctl disable coreos-check-cgroups.service ############################################################################
but when you look at https://docs.fedoraproject.org/en-US/fedora-coreos/kernel-args/ you only see an example of how to initialize a new CoreOS instance with Ignition files with cgroups v2.
Solution:
In order to migrate your system to cgroups v2, run
sudo rpm-ostree kargs --delete=systemd.unified_cgroup_hierarchy
After that, you need to reboot your system in order for the changes to take effect:
sudo systemctl reboot
After the system has rebooted, the error should disappear.