Ubuntu 26.04 LTS – Docker overlay2 fills root disk and containers fail to start

📖 ~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

Administrators observe: Ubuntu 26.04 LTS – Docker overlay2 fills root disk and containers fail to start. Deployments fail with no space left errors.

Environment & Reproduction

Issue appears on hosts with unbounded image retention, logs, and stopped container artifacts.

# Baseline context
lsb_release -a
df -h
du -sh /var/lib/docker/* 2>/dev/null | sort -h
docker system df
sudo journalctl -u docker -n 100 --no-pager

Root Cause Analysis

Unused image layers, build cache, and container logs accumulate in overlay2 until root partition is exhausted.

Quick Triage

Identify top storage consumers and safe reclaim targets.

# Quick triage
docker ps -a --size
docker images --digests
sudo find /var/lib/docker/containers -name '*-json.log' -printf '%s %p
' | sort -nr | head -n 20

Step-by-Step Diagnosis

Correlate container lifecycle with image/cache growth patterns.

# Detailed diagnosis
docker system df -v
sudo du -xh /var/lib/docker/overlay2 | sort -h | tail -n 50
sudo journalctl -u docker --since '7 days ago' --no-pager | tail -n 200

Solution – Primary Fix

Prune unused objects and configure log rotation.

# Primary fix
docker system prune -af --volumes
sudo tee /etc/docker/daemon.json >/dev/null <<'EOF'
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "20m",
    "max-file": "3"
  }
}
EOF
sudo systemctl restart docker

Solution – Alternative Approaches

Move Docker data root to larger dedicated volume.

# Alternative
sudo systemctl stop docker
sudo rsync -aHAX /var/lib/docker/ /mnt/docker-data/
sudo mkdir -p /etc/docker
echo '{"data-root": "/mnt/docker-data"}' | sudo tee /etc/docker/daemon.json
sudo systemctl start docker

Verification & Acceptance Criteria

Disk usage returns to safe level and containers start successfully.

# Verify
df -h
docker system df
docker run --rm hello-world

Rollback Plan

Restore prior daemon config and data-root if container runtime regressions appear.

# Rollback
sudo cp -a /etc/docker/daemon.json /etc/docker/daemon.json.rollback.$(date +%F)
sudo systemctl restart docker

Prevention & Hardening

Apply retention policies for images and logs in CI/CD pipelines.

# Hardening
docker image prune -af
docker builder prune -af
crontab -l | grep -i docker || true

Related Errors & Cross-Refs

Related patterns include failed to register layer and write /var/lib/docker/… no space left on device.

Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.

View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Docker storage driver documentation and Ubuntu container host hardening guidance.