Скачать или смотреть Migrating StatefulSet Pods to New Nodes in Kubernetes

Discover an efficient method to migrate StatefulSet to different nodes in your Kubernetes cluster without unnecessary scaling or pod disruptions.
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Migrating StatefulSet Pods to New Nodes in Kubernetes: A Step-by-Step Guide

When managing Kubernetes clusters, especially in cloud environments like Amazon EKS, there might come a time when you need to change the instance types of your nodes. This could be due to performance improvements, cost considerations, or other reasons. In this post, we'll address a common challenge faced during such migrations: how to effectively migrate StatefulSet pods to different nodes without disruptively scaling your deployments.

The Problem at Hand

Let’s say you have a Kubernetes cluster with 3 nodes running 3 pods of CockroachDB in a StatefulSet. Your goal is to change the instance type for all nodes in your cluster. Here's a general plan that many might consider:

Add a new node and increase the replicas in the StatefulSet to 4.

Decommission one of the old CockroachDB nodes.

Scale down the StatefulSet back to 3 replicas to remove an old pod.

Repeat the process for the remaining nodes.

This approach sounds straightforward, but there's a critical oversight: when you scale down a StatefulSet, Kubernetes will terminate the most recently created pods first. This means your newly added pod may get deleted instead of the older one, complicating the migration.

So, what's the solution?

The Solution: Using ASG and Instance Refresh

Instead of scaling up and down your StatefulSet, we can leverage an Auto Scaling Group (ASG) and perform an instance refresh. Here’s a detailed breakdown:

Step 1: Create a New Launch Template

Copy your existing launch template: Start by duplicating your current ASG launch template.

Upgrade the instance type: Change the instance type in the copied template to the desired configuration.

Step 2: Update the ASG

Point your ASG to the new launch template: In your ASG settings, direct it to utilize your newly created launch template.

Step 3: Execute Instance Refresh

Perform an ASG instance refresh: This will allow the ASG to start replacing instances one at a time.

Healthy percentage: With a cluster of 3 nodes, ensure that a minimum of 90% of the nodes remain healthy. This way, your ASG will handle one instance replacement at a time, preventing downtime.

As the old node is drained, the pod scheduled on that node will enter a pending state for approximately 5-10 minutes before redeploying on the new node.

Benefits of This Method

No unnecessary scaling of StatefulSet: You avoid complicating the StatefulSet management by not needing to increase or decrease replicas manually.

Minimal disruption: Pods are redeployed on new instances automatically, reducing the risk of errors.

Streamlined process: This method leverages Kubernetes and AWS capabilities, making the migration smoother and more predictable.

Final Thoughts

Migrating StatefulSet pods to different nodes in a Kubernetes cluster can be tricky, especially if not done thoughtfully. By using an ASG and an instance refresh, you can successfully achieve a seamless transition to new node types without the challenges of manual scaling and potential pod disruptions.

Remember, understanding the intricacies of your cluster and how Kubernetes manages resources is key to a successful migration strategy.

Feel free to reach out or share your experiences with StatefulSet migrations in the comments below!