Run stat -fc %T /sys/fs/cgroup/ on every Linux node. Kubernetes documents cgroup2fs as the cgroup v2 result and tmpfs as cgroup v1. That command is a node-level detection test, not a complete cluster-readiness verdict: a ready fleet also needs a supported kernel and distribution, a cgroup-v2-capable runtime, and matching systemd cgroup-driver settings in the runtime and kubelet.
The procedure below turns those checks into a repeatable fleet assessment. Record results per node, because one control-plane node or one sample worker cannot prove that a heterogeneous cluster is uniformly running cgroup v2.
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What the first test tells you
On each Linux node, run:
stat -fc %T /sys/fs/cgroup/Interpret the output literally:
| Output | Meaning | What it proves | What it does not prove |
|---|---|---|---|
cgroup2fs |
The mounted hierarchy is cgroup v2. | This node is currently using the unified cgroup v2 filesystem. | It does not prove that the runtime and kubelet are configured compatibly. |
tmpfs |
The mounted hierarchy is cgroup v1 under Kubernetes’ documented check. | This node is currently using cgroup v1. | It does not describe other nodes in the cluster. |
| Anything else or an error | The documented check did not return an expected result. | You need to investigate the node’s mount, permissions, or operating-system state. | It cannot be classified as ready from this command alone. |
Run the command with sufficient permissions on the node itself. The path describes the local Linux kernel and mount, so running it on your workstation, bastion, or only the control plane says nothing about a worker that may run application pods.
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For this guide, a cluster is ready for cgroup v2 when every relevant Linux node passes all of these checks:
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- The node reports
cgroup2fsfrom the filesystem test. - The running kernel is version 5.8 or later.
- The operating-system distribution enables cgroup v2 and the node actually booted into that hierarchy.
- The container runtime supports cgroup v2. Kubernetes lists containerd 1.4 or later and CRI-O 1.20 or later as examples.
- The runtime and kubelet use the systemd cgroup driver, and their effective settings agree.
- The Kubernetes release and its runtime integration are supported for the way you operate the cluster.
This is a deliberately stricter operational definition than “the mount exists.” Kubernetes documentation says the kubelet automatically detects a cgroup-v2 operating system and adapts when the prerequisites are met; automatic detection does not remove the runtime, kernel, or driver requirements.
Step 1: inventory every node and its scope
Start with a node inventory so that a passing result can be tied to a specific machine and software stack. Collect, for each node:
- Node name and role (control plane, worker, or other role used by your distribution).
- Operating system and distribution release.
- Kernel release.
- Kubernetes and kubelet versions.
- Container runtime and runtime version.
- Effective cgroup driver reported or configured for the runtime.
- Effective cgroup driver used by kubelet.
- Whether the node is Linux. Keep any non-Linux nodes in a separate category; cgroups are a Linux-kernel facility and the filesystem test is specified for Linux.
You can use your normal inventory, node-access, or configuration-management system. In a self-managed environment, version commands such as uname -r, kubelet --version, and the runtime’s own version command can help fill the record. Managed Kubernetes services may restrict host access or hide control-plane implementation, so follow the provider’s node-inspection procedure and document which fields are provider-managed.
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Run the check locally
Log in to each Linux node and execute:
stat -fc %T /sys/fs/cgroup/Store the exact output with the node name and timestamp. Do not reduce the result to a single cluster-wide boolean before all nodes have been checked.
Use a controlled remote method
If direct SSH is not your operating model, use the node-access mechanism approved for your cluster to execute the same command in the host namespace. A container that only sees its own isolated filesystem can return a value that does not represent the host; the result must describe the node’s /sys/fs/cgroup/ mount. Your operational tooling should therefore make the host path and host namespace explicit, and it should record which node produced each line.
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Classify mixed results
If one node returns cgroup2fs and another returns tmpfs, the fleet is mixed. You may run cgroup-v2 workloads on the first node, but you cannot claim that the cluster is uniformly ready. Mixed boot settings, image generations, kernels, or runtime configurations are common causes. Keep the nodes separated by result until the underlying differences are understood.
Step 3: verify the kernel and operating-system prerequisites
Kernel version
Kubernetes lists Linux kernel 5.8 or later as a cgroup v2 requirement. Record the running release, not merely the version advertised by an image catalog. For example:
uname -rA kernel newer than 5.8 still needs to be booted with a working cgroup v2 hierarchy and a compatible runtime. Conversely, a node that reports cgroup2fs should still be checked against your distribution’s supported kernel lifecycle and security updates.
Distribution support versus current boot state
Kubernetes documentation gives these distribution examples:
| Distribution or family | Documented cgroup v2 context | How to use the information |
|---|---|---|
| Container-Optimized OS | Since milestone M97 | Confirm the node image and then run the filesystem test. |
| Ubuntu | Since 21.10; 22.04 and later recommended | Verify the actual release and booted hierarchy. |
| Debian | Debian 11 | Check the installed release and node result. |
| Fedora | Fedora 31 | Check the installed release and node result. |
| Arch Linux | Since April 2021 | Check the current image and node result. |
| RHEL and RHEL-like distributions | Since version 9 | Check the exact major release and boot configuration. |
These are examples of distribution support, not proof of the state of a particular machine. Image customization, kernel command-line settings, or an upgrade that retained old boot parameters can leave a supported distribution running cgroup v1. The filesystem result from the running node is authoritative for the hierarchy check.
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When the hierarchy is still v1
A tmpfs result means the node is not currently using cgroup v2. Before changing it, identify how the node image and boot loader are managed. Kubernetes documentation describes manual kernel boot configuration as a possible alternative to distributions that enable cgroup v2 by default, but the exact procedure depends on the operating system and provisioning system. Change the image or boot configuration through that system, reboot in a controlled window, and rerun the test; do not edit an individual node in a way that will be overwritten by the next replacement.
Step 4: verify runtime support and cgroup-driver agreement
Confirm the runtime and version
Record the runtime named by the node and its version. Kubernetes cites containerd 1.4 or later and CRI-O 1.20 or later as examples of runtimes with cgroup v2 support. Treat those figures as compatibility examples, not as a substitute for checking the release documentation for your exact Kubernetes and operating-system combination.
Use the systemd driver consistently
Kubernetes recommends the systemd cgroup driver for cgroup v2. The runtime and kubelet must use compatible settings; kubeadm guidance states that the kubelet’s cgroup-driver value must match the runtime’s driver for kubelet health.
For containerd, the documented option is SystemdCgroup = true. The TOML location of that option differs between containerd 1.x and 2.x. First determine the installed major version, then consult the configuration format for that version and verify the effective setting in the file actually loaded by the running service. Do not paste a 1.x snippet into a 2.x configuration, or the reverse, and assume it was accepted.
After changing a runtime setting, use your normal service-management procedure to reload or restart it, verify that the runtime is healthy, and then restart or reconcile kubelet as required by your operating system. A configuration file that contains the right text but is not read by the active service is not a passing check.
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Check the kubelet side
Inspect the kubelet configuration source used by your distribution or bootstrap tool and identify its effective cgroup-driver value. Depending on how the cluster was created, this may be generated from a kubelet configuration file, a node bootstrap configuration, or a distribution-specific unit. The important test is the effective value used by the running kubelet, not an unused example file.
Compare that value with the runtime’s effective driver. If they differ, stop the migration assessment and correct the mismatch using the cluster’s supported configuration workflow. A node can show cgroup2fs and still fail readiness because the kubelet and runtime disagree.
Step 5: place the test in Kubernetes-version context
cgroup v2 has been stable in Kubernetes since v1.25. The Kubernetes project announced general availability for that release and stated that, on a Linux distribution enabling cgroup v2, the kubelet should adapt without extra configuration when the requirements are met.
Current Kubernetes documentation marks cgroup v1 as deprecated starting in v1.35. That status is a planning signal, not a claim that every existing cluster stops working at that version. Check the exact behavior, defaults, and upgrade notes for the Kubernetes release you operate or plan to adopt.
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When comparing two clusters, record their Kubernetes and kubelet versions alongside the hierarchy and driver results. A node-level pass on an older release does not establish that an upgrade will preserve the same defaults, and a newer release does not eliminate the need to inspect the actual node fleet.
Step 6: produce a defensible readiness result
Use a row per Linux node. A compact report can use these columns:
| Node | Linux? | Filesystem result | Kernel | OS/image | Runtime/version | Kubelet version | Runtime driver | Kubelet driver | Status |
|---|---|---|---|---|---|---|---|---|---|
| worker-a | yes | cgroup2fs |
recorded value | recorded value | recorded value | recorded value | systemd | systemd | ready if all prerequisites pass |
| worker-b | yes | tmpfs |
recorded value | recorded value | recorded value | recorded value | recorded value | recorded value | not ready for a uniform cgroup-v2 fleet |
Use three practical outcomes:
- Ready: the node reports
cgroup2fs, meets the kernel and distribution requirements, runs a supported runtime, and has matching systemd drivers. - Blocked: one or more required checks fail, such as a
tmpfsresult, an old kernel, an unsupported runtime, or a driver mismatch. - Unknown: the field could not be verified, for example because a managed service does not expose host configuration. Unknown is not equivalent to ready; record the provider’s documented guarantee and the inspection limitation.
Declare the cluster fleet-ready only after every relevant Linux node is in the ready category. If your workload is intentionally constrained to a labeled subset, state that scope explicitly and keep non-ready nodes outside the scheduling target.
Troubleshoot the common failure patterns
Only some nodes report cgroup2fs
Compare image generation, distribution release, kernel command line, and runtime version between passing and failing nodes. Replace or reconfigure nodes through the same lifecycle mechanism used to create them, then repeat the per-node test. Do not infer uniformity from the control plane or from the newest node.
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The hierarchy is v2, but kubelet is unhealthy
Check runtime availability and driver agreement first. A cgroup filesystem result does not validate the runtime’s cgroup-v2 implementation or prove that kubelet and runtime are using systemd. Confirm the active runtime configuration, the active kubelet configuration, and service logs using your platform’s standard diagnostics.
The configuration file says systemd, but the node behaves differently
Verify which file and command-line arguments the running service loaded. Containerd’s option path changes between major versions, and generated configuration may override a hand-edited file. Confirm the installed version, inspect the effective configuration, restart through the supported service unit, and rerun the health checks.
A managed cluster does not expose host settings
Separate what you can observe from what the provider guarantees. You may be able to inspect node labels, image versions, or runtime information without host access, while the provider controls kernel and kubelet settings. Use the provider’s cgroup-v2 compatibility statement for that managed release, record the inspection boundary, and avoid claiming a stronger node-by-node result than the available evidence supports.
Migration and change-control checklist
- Inventory all nodes and identify Linux scope, roles, images, kernels, runtimes, kubelets, and drivers.
- Run
stat -fc %T /sys/fs/cgroup/on every Linux node and preserve the raw output. - Resolve every
tmpfs, unknown, or inaccessible result before scheduling cgroup-v2-dependent workloads broadly. - Confirm kernel 5.8 or later and a distribution/image that supports cgroup v2.
- Confirm runtime support, using the documented containerd 1.4+ and CRI-O 1.20+ examples as reference points for compatibility checks.
- Set and verify the systemd cgroup driver for both runtime and kubelet, using the configuration syntax for the installed runtime major version.
- Reboot or restart nodes only through the approved lifecycle process, then repeat the filesystem, runtime, and kubelet checks.
- Record the Kubernetes release context. cgroup v2 is stable from v1.25, while cgroup v1 is documented as deprecated from v1.35; apply the exact upgrade guidance for your release.
- Publish a scoped result that names ready, blocked, and unknown nodes rather than a single unqualified claim.
Why the filesystem command remains useful
The command is small because it answers one precise question: which cgroup filesystem is mounted at the node’s standard cgroup path right now? That makes it a reliable first gate and a useful post-change verification. Its limitation is equally important: cgroup-v2 readiness is an integration property involving the kernel, operating system, runtime, kubelet, and Kubernetes release. Treat the command as the beginning of the assessment, then use the inventory and driver checks to establish whether the whole node fleet can operate consistently.
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Kubernetes documentation states that “The kubelet automatically detects that the OS is running on cgroup v2 and performs accordingly with no additional configuration required.” The surrounding requirements still apply: the operating system, kernel, runtime, and cgroup-driver settings must be compatible. The project’s v1.25 announcement similarly says that the kubelet should adapt without additional configuration when a Linux distribution enables cgroup v2 and the requirements are met. Those statements describe supported automatic behavior; they are not a substitute for checking the nodes you actually run.
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