Summary
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Ubuntu 26.04 will refuse to upgrade systems using cgroup v1; first move containers to cgroup v2.
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Containers will not run on hosts loaded with cgroup v1; old runtimes (Docker
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There is no phasing out, the cut is immediate – check and move containers until the April 23 update.
There are many reasons why I love Linux, and one of my favorites is how it respects legacy hardware. We’ve only just noticed that the Linux kernel has dropped official support for the i486, which was released in 1989, and only because Linus believes that nobody runs Linux on an i486 these days.
However, Ubuntu’s 26.04 update, expected this Thursday, ditches some older software, and it won’t be a slow rollout of older software. Long story short, if your Ubuntu installation uses cgroup v1 for its containers, you will not get the 26.04 update unless you manually migrate your container runtime to cgroup v2 before upgrading.
Ubuntu 26.04 will not arrive on your computer if you are using cgroup v1
There is a fix, but it’s not very nice
With Ubuntu 26.04 arriving on April 23, now is a good time to ensure your systems running Canonical’s distro are ready to go. Part of your preparation should be making sure your legacy apps migrate to 26.04, as it turns out Canonical is giving the ax to specific technologies.
As shown in the document Ubuntu release notes 26.04The OS will suddenly stop some old technologies of the operating system. The biggest one is cgroup v1, which some older container runtimes (eg any version of Docker before 20.10) use. Getting rid of legacy support isn’t great in itself (Red Hat and Fedora got rid of it a few years ago); The real issue is that there is no lag. If you have a cgroup v1 container, Ubuntu 26.04 LTS will refuse to upgrade your system.
Additionally, Ubuntu 26.04 LTS container workloads will not run on the host if loaded with cgroup v1 and will not support containers that require it. So if you want to upgrade to Ubuntu 26.04 LTS, you need to see which cluster group your containers are using and update the ones using v1 to v2.
