Replace root volumes during instance refresh - Amazon EC2 Auto Scaling
How it worksRequirementsStart an instance refresh with Replace Root Volume Limitations
This documentation is a draft for private preview for regions in the AWS European Sovereign Cloud. Documentation content will continue to evolve. Published: December 30, 2025.

Replace root volumes during instance refresh

How it works

Root volume replacement updates your instances by replacing only the root EBS volume while keeping the instance running. This removes the need to launch new instances and avoids potential capacity constraints. This process preserves all other instance resources including:

  • Network interfaces and IP addresses

  • Non-root EBS volumes

  • Instance store volumes and data

  • Security groups and IAM roles

During root volume replacement, your application continues running on the existing instance. The original root volume is detached, a new root volume is created from your specified AMI, and then attached to the same instance. Original root volumes are automatically deleted after successful replacement.

When you use the replace root volume strategy, instances go through these lifecycle states:

  1. ReplacingRootVolume - Root volume replacement begins

  2. ReplacingRootVolume:Wait - Waits for lifecycle hook completion (if configured)

  3. ReplacingRootVolume:Proceed - Proceeds with replacement

  4. RootVolumeReplaced - Replacement completed successfully

After root volume replacement completes, Auto Scaling conducts health checks on updated instances. If root volume replacement fails for an instance, Auto Scaling marks that instance for termination and replaces it with a new instance.

Requirements

  • Your Auto Scaling group must use a mixed instances policy

  • All overrides in the mixed instances policy must specify an ImageId

  • AMIs must contain only a single root volume

  • All instances must match the group's launch template configuration

  • You must start the instance refresh with a desired configuration having a mixed instances policy containing ImageId overrides.

Start an instance refresh with Replace Root Volume

To start an instance refresh with Replace Root Volume (AWS CLI)

Use the following start-instance-refresh command to start an instance refresh from the AWS CLI. You can specify any preferences that you want to change in a JSON configuration file. When you reference the configuration file, provide the file path and name as shown in the following example. 

aws autoscaling start-instance-refresh --cli-input-json file://config.json

Contents of config.json: 

{
  "AutoScalingGroupName": "my-asg",
  "Strategy" : "ReplaceRootVolume",
  "DesiredConfiguration":{
    "MixedInstancesPolicy":{
      "LaunchTemplate": {
        "LaunchTemplateSpecification": {
          "LaunchTemplateId":"my-launch-template",
          "Version": "1"
        },
        "Overrides": [
          {
            "InstanceType":"c5.large",
            "ImageId":"ami-1234example"
          },
          {
            "InstanceType":"m5.large",
            "ImageId":"ami-2345example"
          }
        ]
      }
    }
  },
  "Preferences": {
    "InstanceWarmup": 60,
    "MinHealthyPercentage": 90,
    "AutoRollback": true,
    "ScaleInProtectedInstances": Ignore,
    "StandbyInstances": Ignore
  }
}

If preferences are not provided, default values are used. For more information, see Understand the default values for an instance refresh.

Example output:

{
    "InstanceRefreshId": "08b91cf7-8fa6-48af-b6a6-d227f40f1b9b"
}

Limitations

  • You can’t start an instance refresh with Replace Root Volume if the EC2 Auto Scaling group or the instance refresh desired configuration uses the $Latest or $Default launch template version.

  • You can’t start an instance refresh with Replace Root Volume on an Amazon EC2 Auto Scaling Group if it contains instances whose instance type is not present in the mixed instances policy.

  • During an instance refresh with Replace Root Volume, you can only attach instances with types that are present in the desired configuration's mixed instances policy.

  • You can’t start an instance refresh with Replace Root Volume on an EC2 Auto Scaling Group that has a warm pool.

  • You can't add a warm pool to an Amazon EC2 Auto Scaling group that has an active instance refresh with Replace Root Volume.