Capturing and Reusing a VM Image in Azure

Introduction

Capturing a virtual machine (VM) image in Azure is an essential process when you want to replicate a VM setup across multiple instances. It ensures consistency, saves time, and allows easy deployment of pre-configured environments. This guide explains the entire process, options, and key considerations, including common pitfalls and how to avoid them.

What is a VM Image?

A VM image is a template that contains the operating system, configurations, applications, and data from a virtual machine. You can use it to create new VMs with the same setup, ensuring consistency and efficiency.

Steps to Capture a VM Image in Azure

1. Prepare the VM

Before capturing an image, ensure that the VM is properly prepared:

Generalize the VM

To create a generalized image suitable for scaling and multiple deployments, the VM must be generalized to remove machine-specific information like hostnames and user accounts.

• For Windows VMs:

  1. Connect to the VM via Remote Desktop (RDP).

  2. Open Command Prompt as an administrator.

  3. Run the following command:

     sysprep /generalize /shutdown

  4. In the Sysprep tool, select the "Enter System Out-of-Box Experience (OOBE)" option and check the "Generalize" checkbox.

  5. Click OK to start the process. The VM will shut down automatically when completed.

• For Linux VMs:

  1. Connect to the VM using SSH.

  2. Run the following commands:

     sudo waagent -deprovision+user
     sudo shutdown -h now

  3. The VM will shut down after completing the generalization process.

Note: Generalization is mandatory for creating scalable, reusable images. If you skip this step, Azure will create a specialized image, retaining all machine-specific data.

2. Capture the Image

Once the VM is generalized and shut down:

1. Go to the Azure portal.

2. Navigate to the VM you want to capture.

3. Click on Capture in the VM settings.

4. Provide a name for the image and select the resource group where you want to save it.

5. Check the box for "The virtual machine has been generalized" (Azure assumes you’ve completed the generalization process).

6. Optionally, select "Automatically delete this virtual machine after creating the image."

   • This will delete the generalized VM after the image is captured, as it cannot be used further.

7. Confirm and start the image capture process.

Options When Capturing a VM Image

1. Generalized vs. Specialized Image

• Generalized Image:

  • Suitable for scalable deployments.

  • Requires the VM to be generalized beforehand (e.g., using Sysprep or waagent).

• Specialized Image:

  • Captures the exact state of the VM, including user accounts and configurations.

  • No generalization is required.

  • Best for backup purposes or specific one-off deployments.

2. Azure Compute Gallery:

• Used for managing and versioning images.
• Allows sharing across subscriptions and regions.

   

3. Delete or Retain the VM

• Delete After Capture:

  • Recommended if the VM is generalized since it can no longer be used.

• Keep the VM:

  • Useful for specialized images or when you need to retain the original VM for any reason.

Important Considerations

1. Generalization is Critical

If we plan to use the image for scalable deployments, generalization removes machine-specific settings. Skipping this step results in a specialized image, which may cause conflicts in multi-instance scenarios.

2. Downtime is Required

Generalization and image capture require the VM to be shut down. Plan for this downtime, especially in production environments.

3. Licensing

Ensure compliance with licensing terms for the OS and any installed software. Some licenses might require specific configurations or approvals for image creation.

4. Testing the Image

After creating the image, deploy a test VM to confirm that the image works as expected.

Common Questions

Q: What happens if I forget to generalize the VM?

If we skip generalization, the image will be specialized and retain all machine-specific configurations, and it could lead to conflicts during CM creation.

Q: Can I generalize a VM after it is deleted?

No, once the VM is deleted, you cannot generalize it. Generalization must be done before capturing the image.

Q: What is the difference between a snapshot and an image?

• A snapshot captures the current state of a VM’s disk.

• An image is a reusable template for creating new VMs.

Conclusion

Capturing a VM image in Azure involves careful preparation and consideration. By generalizing the VM, choosing the appropriate image type, and planning for downtime, we can ensure smooth deployments. Whether we're creating scalable application environments or backing up specific configurations, Azure VM images provide a powerful and flexible solution.