Moving a Kubernetes node from cgroup v1 to cgroup v2 is primarily an operating-system and compatibility change, not a workload migration. The kubelet normally detects v2 automatically, but the node image, Linux kernel, container runtime, cgroup driver, monitoring and security agents, and applications that inspect cgroup files must all be ready. Kubernetes now treats v1 as a deprecation concern: beginning with Kubernetes 1.35, a kubelet on a v1 node does not start by default unless the temporary failCgroupV1: false override is set.
What cgroup v1 and v2 do in a Kubernetes node
Linux control groups (cgroups) organize processes into groups and apply resource controls such as CPU time, memory limits, process counts and accounting. Kubernetes uses them on every Linux node to enforce pod and container requests and limits. Both the kubelet and the container runtime must talk to the same control-group system; if they do not, resource enforcement and node stability can suffer. The Kubernetes documentation explains this relationship in its Container Runtimes guide.
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cgroup v1 exposes separate, often independently mounted hierarchies for controllers such as memory, CPU and devices. cgroup v2 replaces that model with a unified hierarchy and a newer interface. Kubernetes support for v2 has been stable since version 1.25. The change affects how the operating system presents resource files, how services are delegated to child cgroups, and which tools can read usage and pressure information. It does not require rewriting ordinary Kubernetes manifests: a pod’s requests and limits remain Kubernetes API fields.
Key differences operators need to plan for
| Area | cgroup v1 | cgroup v2 | Operational consequence |
|---|---|---|---|
| Hierarchy and API | Multiple controller hierarchies and controller-specific file layouts. | One unified hierarchy with a common control-group interface and safer subtree delegation. | Scripts and agents that expect v1 paths or filenames can fail even when pods run normally. |
| Resource accounting | Accounting and controls vary by controller and implementation. | More consistent accounting, unified delegation rules and additional facilities such as Pressure Stall Information (PSI). | Capacity and saturation dashboards may need v2-aware collectors; do not infer a performance gain without measurements for your workload. |
| Node software compatibility | Older monitoring, security and runtime components commonly assume v1 files. | Components must understand the unified hierarchy and v2 filenames and semantics. | Inventory anything that reads /sys/fs/cgroup directly before draining a node. |
| Kernel and distribution | Available on older Linux distributions and kernels. | Requires a distribution that enables v2 and a sufficiently new kernel. | Changing the kubelet setting alone cannot provide v2; the node image and boot configuration must support it. |
| Runtime and kubelet driver | Legacy systems often used the cgroupfs driver. | Systemd-based nodes should normally use the systemd driver in both kubelet and runtime. | Both components must use the same driver or resource views can diverge. |
| Kubernetes lifecycle | Still usable only as a transition path in releases that permit it. | The strategic direction for supported Linux nodes. | Plan the move before upgrading to a release that rejects v1 by default. |
The kernel’s authoritative description of the unified interface is the Linux cgroup v2 documentation. Kubernetes describes the platform-specific behavior in About cgroup v2.
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What Kubernetes does automatically—and what it does not
When the operating system is already running cgroup v2, the kubelet detects that mode and adapts without an additional Kubernetes cgroup-version switch. That automatic detection does not upgrade the operating system, repair an incompatible runtime, or translate third-party software that opens v1 files. The node still needs a supported kernel, distribution, runtime and driver configuration.
Pod specifications also do not change merely because the node uses v2. CPU and memory requests and limits continue to be declared in the usual way. The observable changes are below the Kubernetes API: different files, different delegation behavior, new accounting capabilities and, where configured, a different way of applying swap limits.
Prerequisites before converting a node
Use a v2-capable distribution and kernel
Kubernetes documents Linux kernel 5.8 or later as the cgroup v2 requirement. The distribution must enable v2 and boot with it; a kernel that contains v2 support is not sufficient if the vendor image leaves v2 disabled. Treat the documented version as a minimum, not a guarantee that every combination of distribution, runtime and Kubernetes release is supported. Check the exact node-image support statement from your Linux vendor and Kubernetes provider.
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The Kubernetes documentation gives containerd 1.4 or later and CRI-O 1.20 or later as example minimum runtime versions for v2. Verify the runtime’s support matrix for the OS and Kubernetes version you operate. Upgrade the runtime as part of a tested node-image change rather than changing the cgroup mode on an otherwise frozen image.
Align the cgroup driver
The kubelet and runtime must select the same cgroup driver. On a systemd-based Linux distribution, Kubernetes recommends systemd, because systemd is already managing services and their control groups. Running systemd and a separate cgroup manager against the same hierarchy can create conflicting resource views and instability under pressure. The driver setting is independent of whether the kernel is v1 or v2, but v2 migrations are a good time to eliminate a mismatch.
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Check swap policy separately
cgroup v2 can expose a memory.swap.max limit to the runtime, but that capability does not turn swap on for pods automatically. Linux kubelet behavior is still governed by its swap configuration; with swap enabled, kubelet does not start by default unless it has been configured to tolerate that condition. Follow the Kubernetes swap memory management documentation and validate both kubelet and runtime settings. Decide whether swap is disabled, tolerated for the node, or deliberately available to workloads, and document that policy before rollout.
How to identify the current mode and driver
Run the following on each Linux node, preferably through the same privileged maintenance path used for other node checks:
stat -fc %T /sys/fs/cgroup/cgroup2fsindicates cgroup v2.tmpfsindicates cgroup v1 in the Kubernetes instructions.
Confirm the result against the node distribution’s documentation, especially on images that mount compatibility hierarchies. Then inspect the kubelet configuration and the runtime configuration for their cgroup-driver value. A node is not ready for migration if one says systemd and the other says cgroupfs.
For a view of the systemd hierarchy and the services placed in it, use:
systemd-cgls
systemd-cgtop
systemctl list-units 'kube*' --type=sliceThese commands help reveal whether kubelet, the runtime and other node services are in the slices you expect. They do not replace checking the runtime’s own configuration or the files used by an agent.
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Inventory software that may break on v2
The highest migration risk is software that reads cgroup files directly rather than using a runtime or operating-system API. Create an inventory for every daemon, sidecar and host application that has access to /sys/fs/cgroup.
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Check node exporters, custom metrics collectors, autoscaling integrations and dashboards for v1-only paths such as controller-specific memory or CPU directories. Update them to versions that understand the unified hierarchy and verify that reported usage, limits and throttling still match the runtime’s view. If cAdvisor is deployed as a standalone DaemonSet, Kubernetes specifically advises version 0.43.0 or later for cgroup v2.
Security and observability agents
Host intrusion sensors, policy agents, profilers and log collectors can depend on v1 filenames, mount discovery or delegation behavior. Ask each vendor for its cgroup v2 support statement for the exact agent version, kernel and distribution. Test startup, rule loading and event collection after the node reboots in v2 mode; an agent that starts but silently loses container attribution is not compatible enough.
Application runtimes
Some language runtimes, databases and embedded libraries inspect cgroups to size worker pools or report container limits. Verify their documented v2 support and test the limit-detection path under a deliberately constrained pod. A process that falls back to host memory or CPU counts can over-allocate even though Kubernetes is enforcing the pod limit correctly.
In-house scripts
Search images, DaemonSets, systemd units and host scripts for hard-coded paths under /sys/fs/cgroup. Replace path assumptions with a supported library or runtime interface where possible. If direct inspection is unavoidable, implement explicit v1 and v2 detection and test the files and permissions on both modes.
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A controlled node-by-node migration procedure
Use a surge or replacement strategy where possible: build a known-good v2 node image, add new nodes, move workloads, then retire v1 nodes. In-place boot changes are possible but increase recovery complexity because a failed reboot can remove the node from service.
- Record the baseline. Capture the node’s cgroup mode, kernel and distribution release, kubelet and runtime versions, cgroup-driver values, swap state, systemd slices, and the versions of monitoring and security agents. Save representative pod CPU, memory, throttling and restart metrics so post-migration checks have a comparison point.
- Validate the image in a non-production pool. Boot the target distribution with cgroup v2 enabled, confirm the
statresult, and install the supported runtime and kubelet versions. Do not proceed if the runtime cannot create a test pod or if either component reports a driver mismatch. - Update dependent agents and images. Install v2-capable versions of collectors and security software, including cAdvisor 0.43.0 or later when it is a standalone DaemonSet. Rebuild any image that contains a v1-only cgroup reader.
- Exercise resource enforcement. Run test pods with CPU and memory requests and limits, observe runtime and kubelet events, and confirm that metrics show the expected limit and throttling behavior. Include a pod that approaches its memory limit so that you test an actual enforcement path rather than only pod startup.
- Drain the production node. Apply the normal disruption-budget-aware drain procedure, respecting workload replication and local-storage exceptions. Keep the old node available until the replacement has passed health checks.
- Join or reboot the node in v2 mode. Start kubelet and the runtime only after their driver configuration is aligned. Check node readiness, runtime health and systemd slices before scheduling ordinary workloads.
- Run post-join checks. Verify that DaemonSets are ready, logs contain no cgroup mount or permission errors, and metrics identify containers correctly. Compare resource usage and eviction signals with the baseline rather than assuming identical file names or counter labels.
- Canary real workloads. Return a small, representative set of services first. Watch latency, restarts, OOM events, throttling, autoscaler inputs and security alerts through a normal peak period. Expand the pool only after those signals remain correct.
- Retire the v1 node. Keep a documented rollback image or spare capacity until the canary and the first production cycle are complete. Then remove the old node so that future upgrades cannot accidentally depend on its v1 behavior.
Understanding the Kubernetes 1.35 deprecation change
Kubernetes marks cgroup v1 as deprecated starting in version 1.35. In that release, kubelet does not start on a v1 node by default. The documented temporary opt-out is failCgroupV1: false in the kubelet configuration. This setting is a bridge for upgrades and troubleshooting, not a long-term migration plan; removal timing follows the Kubernetes deprecation policy.
Plan the work against the Kubernetes version that will run on the node, not only the version currently installed. A cluster that is healthy on v1 today can fail to rejoin after a node-image refresh or control-plane upgrade if the new kubelet applies the default v1 failure behavior.
Swap and memory behavior on cgroup v2
Swap is a policy decision with two independent parts: whether kubelet permits swap on the node and whether the runtime applies a swap limit to a container. On v2, the runtime can use memory.swap.max, but the setting must be configured and verified. A v2 node with swap enabled does not automatically provide swap to every pod, and a pod limit does not by itself describe how much swap it may consume.
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Troubleshooting common migration failures
Kubelet refuses to start after the upgrade
On a v1 node running a Kubernetes release with the new default, this is expected behavior rather than a random service failure. Confirm the mode with stat -fc %T /sys/fs/cgroup/. The durable fix is to move the node to a v2-capable image; the temporary documented compatibility setting is failCgroupV1: false. Check the kubelet version and configuration file actually used by the service before changing anything.
Pods start but limits or metrics look wrong
Check that kubelet and runtime use the same driver, then inspect the runtime’s cgroup placement and the collector’s v2 support. A dashboard that still reads v1 paths can show zeros or host-wide values while enforcement is working. Compare direct runtime information with the collector output and update the collector before changing pod specifications.
The runtime cannot create a sandbox
Review runtime logs for controller, mount or delegation errors. Confirm that the distribution booted the intended hierarchy, that the runtime version supports it, and that systemd and the runtime are not managing the same subtree through different drivers. Recreate the node from the tested image if the boot configuration is ambiguous.
A security or observability agent loses container attribution
Inspect the agent’s startup log and its cgroup discovery method. Upgrade to a v2-compatible release, grant only the documented host access, and test attribution on a pod with a known namespace and container ID. Do not treat “the daemon is running” as proof that its events are still correctly associated.
Workloads show unexpected memory pressure
First distinguish a changed measurement from changed enforcement. Compare kubelet eviction events, runtime counters, PSI and application-level memory metrics. Check swap policy and whether an application runtime changed its container-size detection. Roll back the canary or cordon the node while correcting the component that changed its interpretation.
Managed Kubernetes and provider-owned nodes
In a managed service, the provider may control the node image, boot parameters, kubelet flags and runtime version. Ask for the provider’s cgroup v2 adoption schedule, supported node images, upgrade behavior and rollback procedure. The general Kubernetes documentation does not establish a universal timeline or command sequence for every provider, so do not infer one from a self-managed cluster. You still own workload compatibility: inspect DaemonSets, admission-installed agents and applications that read host cgroup files.
Migration decision checklist
- Is the target distribution explicitly enabling cgroup v2, and is the kernel at least 5.8?
- Is the container runtime version supported for the target OS and Kubernetes release?
- Do kubelet and runtime use the same cgroup driver, preferably
systemdon a systemd-based node? - Have all host agents, DaemonSets and application runtimes that inspect cgroup files been identified and upgraded?
- Does standalone cAdvisor meet the documented v2 guidance of version 0.43.0 or later?
- Have swap behavior and the desired
memory.swap.maxpolicy been tested separately? - Have you tested resource enforcement, metrics, autoscaling inputs, evictions and security events on a canary node?
- Do you have spare capacity and a rollback image while the first production pool is observed?
- If the cluster is managed, have you obtained provider-specific image and upgrade instructions?
Bottom line for cluster operators
cgroup v2 is a unified, better-defined Linux control-group interface that Kubernetes supports natively, but migration success depends on the node software around Kubernetes. Build or select a v2-enabled image, meet the kernel and runtime requirements, align the kubelet and runtime drivers, update direct cgroup readers, and make swap behavior explicit. Migrate with replacement nodes and canaries where possible. Treat the Kubernetes 1.35 v1 deprecation as an upgrade deadline signal, while using failCgroupV1: false only as a temporary bridge.
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