设置DeleteOnTermination=true创建EC2实例失败问题排查

The Problem #

You’re trying to set DeleteOnTermination to true for all volumes in your CloudFormation template by adding the BlockDeviceMappings section, but you’re hitting this error:

Invalid value '/dev/sdf' for unixDevice. Attachment point /dev/sdf is already in use

Let’s break down why this is happening and how to fix it.

Why This Error Occurs #

Your template has two conflicting configurations for the /dev/sdf device:

1. The Volumes section mounts a pre-created EBS volume (Volume67430dc919ab486a887388695720e574) to /dev/sdf.
2. The BlockDeviceMappings section tries to create a new 100GB EBS volume and attach it to the same /dev/sdf path.

EC2 instances can’t attach two volumes (or a pre-created + new volume) to the same device path—this is what’s causing the "already in use" conflict.

How to Fix It #

You have two valid solutions depending on whether you need to keep the pre-created volume or not:

Solution 1: Keep the Pre-Created Volume & Set DeleteOnTermination

If you want to retain the standalone AWS::EC2::Volume resource, update your BlockDeviceMappings to reference this existing volume instead of creating a new one. You can still set DeleteOnTermination here:

• Remove the /dev/sdf entry in BlockDeviceMappings that uses the Ebs property (which creates a new volume)
• Add a new /dev/sdf entry that links to your pre-created volume via VolumeId, and include the DeleteOnTermination setting
• Remove the Volumes section entirely—BlockDeviceMappings handles both volume attachment and deletion policy when using VolumeId

Here’s the modified AWS::EC2::Instance Properties section:

"Properties": {
  "InstanceType": "t2.micro",
  "BlockDeviceMappings": [
    {
      "DeviceName": "/dev/sda1",
      "Ebs": {
        "VolumeSize": "50",
        "DeleteOnTermination": "true"
      }
    },
    {
      "DeviceName": "/dev/sdf",
      "VolumeId": { "Ref": "Volume67430dc919ab486a887388695720e574" },
      "Ebs": {
        "DeleteOnTermination": "true"
      }
    }
  ],
  "ImageId": "ami-6871a115",
  "KeyName": "${aws_key_pair.KP6a0efc01a84c4103b648b57b425b0ca2.key_name}",
  "NetworkInterfaces": [
    {
      "AssociatePublicIpAddress": "true",
      "DeviceIndex": "0",
      "SubnetId": "subnet-07f722e24335cea6f",
      "GroupSet": [ "sg-0ed40ed2e6af3db09" ]
    }
  ],
  "Tags": [
    { "Key": "Name", "Value": "jagaranteamvoltest" },
    { "Key": "vid", "Value": "c42a7c2e-ed09-4a79-acd8-51bdc9772a13" }
  ]
}

Solution 2: Remove the Pre-Created Volume, Use Only BlockDeviceMappings

If you don’t need to manage the volume independently of the instance, simplify your template:

• Delete the Volumes section from the instance properties
• Delete the standalone Volume67430dc919ab486a887388695720e574 resource (and any related dependencies if applicable)
• Keep your original BlockDeviceMappings for /dev/sdf—it will create the volume directly with your desired size and DeleteOnTermination setting

This is the cleaner option if the volume doesn’t need to exist separate from the instance.

Key Takeaway #

Never mix Volumes and BlockDeviceMappings for the same device path. BlockDeviceMappings is the only section where you can set DeleteOnTermination, so use it to either:

• Create and attach new volumes, or
• Attach existing volumes while defining their deletion policy

内容的提问来源于stack exchange，提问作者nightfly