Blog post about cgroup v1 support moving to maintenance mode / KEP-4569

Signed-off-by: Harshal Patil <harpatil@redhat.com>

content/en/blog/_posts/2024-08-15-moving-cgroup-v1-support-maintenance-mode-kubernetes-1-31.md

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+---
+layout: blog
+title: "Kubernetes 1.31: Moving cgroup v1 Support into Maintenance Mode"
+date: 2024-08-13
+slug: kubernetes-1-31-moving-cgroup-v1-support-maintenance-mode
+author: Harshal Patil
+---
+
+As Kubernetes continues to evolve and adapt to the changing landscape of
+container orchestration, the community has decided to move cgroup v1 support
+into [maintenance mode](#what-does-maintenance-mode-mean) in v1.31.
+This shift aligns with the broader industry's move towards cgroup v2, offering
+improved functionalities: including scalability and a more consistent interface.
+Before we dive into the consequences for Kubernetes, let's take a step back to
+understand what cgroups are and their significance in Linux.
+
+## Understanding cgroups
+
+[Control groups](https://man7.org/linux/man-pages/man7/cgroups.7.html), or
+cgroups, are a Linux kernel feature that allows the allocation, prioritization,
+denial, and management of system resources (such as CPU, memory, disk I/O,
+and network bandwidth) among processes. This functionality is crucial for
+maintaining system performance and ensuring that no single process can
+monopolize system resources, which is especially important in multi-tenant
+environments.
+
+There are two versions of cgroups:
+[v1](https://docs.kernel.org/admin-guide/cgroup-v1/index.html) and
+[v2](https://docs.kernel.org/admin-guide/cgroup-v2.html). While cgroup v1
+provided sufficient capabilities for resource management, it had limitations
+that led to the development of cgroup v2. Cgroup v2 offers a more unified and
+consistent interface, on top of better resource control features.
+
+## Cgroups in Kubernetes
+
+For Linux nodes, Kubernetes relies heavily on cgroups to manage and isolate the
+resources consumed by containers running in pods. Each container in Kubernetes
+is placed in its own cgroup, which allows Kubernetes to enforce resource limits,
+monitor usage, and ensure fair resource distribution among all containers.
+
+### How Kubernetes uses cgroups
+
+**Resource Allocation**
+: Ensures that containers do not exceed their allocated CPU and memory limits.
+
+**Isolation**
+: Isolates containers from each other to prevent resource contention.
+
+**Monitoring**
+: Tracks resource usage for each container to provide insights and metrics.
+
+## Transitioning to Cgroup v2
+
+The Linux community has been focusing on cgroup v2 for new features and
+improvements. Major Linux distributions and projects like
+[systemd](https://github.com/systemd/systemd/issues/30852) are
+transitioning towards cgroup v2, which provides several benefits over cgroup
+v1, such as Unified Hierarchy, Improved Interface, Better Resource Control,
+[cgroup aware OOM killer](https://github.com/kubernetes/kubernetes/pull/117793),
+[rootless support](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2033-kubelet-in-userns-aka-rootless/README.md#cgroup) etc.
+
+Given these advantages, Kubernetes is also making the move to embrace cgroup
+v2 more fully. However, this transition needs to be handled carefully to avoid
+disrupting existing workloads and to provide a smooth migration path for users.
+
+## Moving cgroup v1 support into maintenance mode
+
+### What does maintenance mode mean?
+
+When cgroup v1 is placed into maintenance mode in Kubernetes, it means that:
+
+1. **Feature Freeze**: No new features will be added to cgroup v1 support.
+2. **Security Fixes**: Critical security fixes will still be provided.
+3. **Best-Effort Bug Fixes**: Major bugs may be fixed if feasible, but some
+issues might remain unresolved.
+
+### Why move to maintenance mode?
+
+The move to maintenance mode is driven by the need to stay in line with the
+broader ecosystem and to encourage the adoption of cgroup v2, which offers
+better performance, security, and usability. By transitioning cgroup v1 to
+maintenance mode, Kubernetes can focus on enhancing support for cgroup v2
+and ensure it meets the needs of modern workloads. It's important to note
+that maintenance mode does not mean deprecation; cgroup v1 will continue to
+receive critical security fixes and major bug fixes as needed.
+
+## What this means for cluster administrators
+
+Users currently relying on cgroup v1 are highly encouraged to plan for the
+transition to cgroup v2. This transition involves:
+
+1. **Upgrading Systems**: Ensuring that the underlying operating systems and
+container runtimes support cgroup v2.
+2. **Testing Workloads**: Verifying that workloads and applications function
+correctly with cgroup v2.
+
+## Further reading
+
+- [Linux cgroups](https://man7.org/linux/man-pages/man7/cgroups.7.html)
+- [Cgroup v2 in Kubernetes](/docs/concepts/architecture/cgroups/)
+- [Kubernetes 1.25: cgroup v2 graduates to GA](/blog/2022/08/31/cgroupv2-ga-1-25/)