How to fix sync loop hang in Kubernetes

KubernetesADVANCEDHIGH

A sync loop hang occurs when the kubelet's main reconciliation loop becomes blocked or unresponsive. The node stops processing pod changes, leaving pods in inconsistent states and blocking cluster operations.

What this error means

The kubelet runs a sync loop that: 1. Monitors desired pod state from the API server 2. Reconciles actual pod state on the node 3. Creates/updates/deletes containers to match desired state When this loop hangs (blocks indefinitely), the kubelet: - Stops processing new pods - Stops cleaning up terminated pods - Cannot respond to API server requests - Appears to be frozen but may not crash Common causes: deadlocks in kubelet code, I/O operations blocking, network timeouts, or resource exhaustion.

How to fix "sync loop hang"

1Identify sync loop hang in kubelet logs

SSH into the affected node and check kubelet status:

bash

ps aux | grep kubelet
sudo systemctl status kubelet

Check kubelet logs for hang indicators:

bash

sudo journalctl -u kubelet --no-pager | tail -100

Look for:
- No log entries for extended period (> 30 seconds)
- Repeating error messages (stuck in retry loop)
- "sync pod" or "sync loop" keywords
- "context deadline exceeded"

Check if kubelet process is responsive:

bash

kill -0 <kubelet-pid>  # Exit code 0 = process exists
strace -p <kubelet-pid> 2>&1 | head -10  # See what it's doing

If strace shows I/O wait or network system calls, the process is blocked.

2Check node resources and I/O state

Verify node has sufficient resources:

bash

free -h  # Memory
df -h  # Disk space
blk-iosat  # I/O saturation
top -b -n1 | head -20  # CPU and process info

Check disk I/O specifically:

bash

sudo iostat -x 1 5  # 5 reports, 1 second intervals

Look for util% near 100% (disk saturation).

Check file descriptor limit:

bash

ulimit -n
lsof | wc -l  # Open files count

If disk is full or heavily I/O bound:

bash

sudo lsof | sort -k7 -n | tail -20  # Files by size
sudo du -sh /var/lib/kubelet/* | sort -rh | head

Free space if needed:

bash

sudo rm -rf /var/lib/kubelet/pods/*/volumes/*  # Temporary volumes

3Test API server connectivity from kubelet

The sync loop communicates with the API server. Test connectivity:

bash

curl -k https://<api-server-ip>:6443/api/v1/nodes

If timeout:
- Network partition or firewall issue
- API server overloaded
- kubelet network stuck in TIME_WAIT

Reset network connections:

bash

sudo systemctl restart networking  # On some distros
sudo ip route flush cache

Check kubelet request timeout setting:

bash

ps aux | grep kubelet | grep -o "--request-timeout=[^ ]*"

Default is 70s. If API server is slow, increase it:

bash

# Edit kubelet config
sudo nano /etc/sysconfig/kubelet
# Add: --request-timeout=120s
sudo systemctl restart kubelet

4Check container runtime responsiveness

The sync loop calls container runtime frequently. Verify runtime is responsive:

containerd:

bash

sudo ctr -a /run/containerd/containerd.sock version
sudo ctr -a /run/containerd/containerd.sock tasks list

Docker:

bash

sudo docker ps
sudo docker stats --no-stream

If runtime is slow/hung:
- Check runtime logs (see kubelet-runtime-error article)
- Restart runtime:

bash

sudo systemctl restart containerd  # or docker, crio

- But wait for kubelet to sync first; don't restart both simultaneously

Monitor runtime operations:

bash

sudo strace -p <runtime-pid> -e trace=network,open,openat 2>&1 | head -20

5Review recent kubelet code changes or upgrades

If sync loop hang started after a recent change:

Check current kubelet version:

bash

kubelet --version
kubectl get nodes -o wide  # Shows version

Check release notes for known issues:
- https://github.com/kubernetes/kubernetes/releases
- Search for "sync loop" and "hang"

Common problematic versions have patches. Upgrade if available:

bash

# For kubeadm clusters
sudo kubeadm upgrade plan
sudo kubeadm upgrade node

For managed services (EKS, GKE, AKS), check if a node update is available:

bash

# AWS EKS
aws eks describe-nodegroup --cluster-name <cluster> --nodegroup-name <group>

# GKE
gcloud container node-pools update <pool> --cluster <cluster> --enable-upgrade-on-boot

6Monitor kubelet with verbose logging

Enable debug logging to see what kubelet is doing:

bash

sudo nano /etc/sysconfig/kubelet
# Add: --v=4  # Increase verbosity (1-4)
sudo systemctl restart kubelet

Then monitor logs in real-time:

bash

sudo journalctl -u kubelet -f --output=short-iso

High verbosity will show:
- Pod sync operations
- API server requests
- Container operations
- Lock acquisitions

Watch for:
- Stuck on one operation for > 30 seconds
- "Timed out waiting for..." messages
- Panic or fatal errors

After debugging, reduce verbosity to avoid log spam:

bash

sudo nano /etc/sysconfig/kubelet
# Change: --v=2  # Or remove
sudo systemctl restart kubelet

7Force restart kubelet as emergency recovery

If sync loop is hung and blocking cluster operations:

bash

# Cordon node to prevent new pods
kubectl cordon <node-name>

# Force-restart kubelet (may lose data on tmpfs volumes)
sudo systemctl stop kubelet
sleep 5
sudo systemctl start kubelet

# Monitor recovery
sudo journalctl -u kubelet -f
kubectl get nodes -w

If kubelet won't stop:

bash

sudo killall -9 kubelet

Then restart:

bash

sudo systemctl start kubelet

Pods affected by the restart may show errors. Let the node recover:

bash

kubectl uncordon <node-name>
kubectl get pods -A --field-selector spec.nodeName=<node-name> -w

After recovery, investigate root cause (Steps 1-5).

8Replace node if sync loop hangs repeatedly

If the node exhibits sync loop hangs consistently:

1. Cordon the node:

bash

kubectl cordon <node-name>

2. Drain workloads:

bash

kubectl drain <node-name> --ignore-daemonsets --delete-emptydir-data

3. Remove the node from cluster:

bash

kubectl delete node <node-name>

4. Terminate the actual node:

bash

# AWS
aws ec2 terminate-instances --instance-ids <instance-id>

# GCP
gcloud compute instances delete <instance-name>

# Azure
az vm delete --resource-group <group> --name <vm-name>

5. New node will auto-join (if cluster auto-scaling is enabled):

bash

kubectl get nodes -w

For on-prem, decommission and replace the physical/VM hardware.

How to fix sync loop hang in Kubernetes

What this error means

Typical symptoms

Common causes

How to fix "sync loop hang"

Advanced notes

Related errors

Official resources & further reading