RHEL 7 Docker daemon fails to start because kernel cgroup features are missing

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Container workloads stop because dockerd cannot start. CI/CD jobs fail and application services depending on containers become unavailable, often after patching or kernel argument changes.

Environment & Reproduction

Appears on older RHEL 7 kernels or misconfigured boot parameters where required cgroup controllers are disabled. Reproduce by removing cgroup options and restarting docker with systemctl.

Root Cause Analysis

Docker runtime expects specific cgroup capabilities; when unavailable, initialization halts. Kernel, initramfs, and daemon configuration drift can create incompatible startup assumptions.

Quick Triage

Check systemctl status docker, inspect journalctl -u docker, and verify mounted cgroup controllers. Confirm kernel version and boot args from /proc/cmdline before changing daemon settings.

Step-by-Step Diagnosis

Map daemon error messages to missing controllers, compare host baseline with working nodes, and validate storage driver compatibility. Use journalctl and system logs to confirm exact startup blocker.

Solution – Primary Fix

Align kernel boot parameters and docker daemon config, update/reboot into supported kernel if needed, then systemctl daemon-reload and systemctl restart docker. Verify service wrappers report healthy state.

Solution – Alternative Approaches

Temporarily run critical workloads on alternate hosts, pin to compatible docker version, or migrate to supported runtime stack while planning platform upgrades.

Verification & Acceptance Criteria

docker info must complete without cgroup warnings and containers should start normally. journalctl should show stable daemon startup with no recurring initialization errors.

Rollback Plan

Revert kernel arg changes and daemon configuration to last-known good state, then reboot if required. Restore workloads from standby hosts if local recovery is delayed.

Prevention & Hardening

Maintain tested kernel/runtime compatibility matrix, validate boot args in config management, and monitor docker startup errors continuously. Document service and systemctl procedures for controlled recovery.

Related Errors & Cross-Refs

Related faults include failed to start daemon: devices cgroup isn’t mounted. Cross-reference grub changes, kernel package updates via yum, and host hardening settings including SELinux.

Related tutorial: View the step-by-step tutorial for rhel-7.

View all rhel-7 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Consult Docker engine docs, RHEL container guidance, and kernel cgroup references. Add upgrade and rollback patterns to platform operations documentation.