Ubuntu will not upgrade you
Ubuntu 26.04 LTS removed cgroup v1 from the kernel and systemd no longer mounts the legacy hierarchy, so hosts whose container runtime still depends on v1 are blocked from upgrading. One stat command tells you which hierarchy you are on before you start.
Twenty-six oh four refuses the upgrade if your containers still use the old cgroup hierarchy.
Upgrade blockers usually show up halfway through, when the machine is already in a strange state and you are reading a log to work out what stopped.
This one you can check in advance, in one command. Ask what filesystem is mounted at the cgroup path.
The answer here is cgroup two f s. That is the unified hierarchy, and it is what the new release requires.
The controllers file lists what the kernel exposes through it. If instead you find the old per-controller directories, your runtime is still on version one.
Version one is gone from the kernel in the new release, and systemd no longer mounts it. There is no fallback flag. The migration happens before the upgrade or the upgrade does not happen.
Check the hierarchy before you plan the upgrade, not during it.
#linux#ubuntu#docker#devops#sysadmin
❯ Check the hierarchy before you plan the upgrade, not during it.