storage_account_type should not be needed for ephemeral OS disks

[nathanweeks]

Is there an existing issue for this?

• I have searched the existing issues

Community Note

• Please vote on this issue by adding a 👍 reaction to the original issue to help the community and maintainers prioritize this request
• Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
• If you are interested in working on this issue or have submitted a pull request, please leave a comment and review the contribution guide to help.

Terraform Version

1.6.4

AzureRM Provider Version

3.96.0

Affected Resource(s)/Data Source(s)

azurerm_linux_virtual_machine, azurerm_windows_virtual_machine, azurerm_linux_virtual_machine_scale_set, azurerm_windows_virtual_machine_scale_set

Terraform Configuration Files

# https://raw.githubusercontent.com/hashicorp/terraform-provider-azurerm/v3.96.0/examples/virtual-machines/linux/ephemeral-os-disk/main.tf
# with variables.tf merged and storage_account_type line commented-out
#########################################
# Copyright (c) HashiCorp, Inc.                                                     
# SPDX-License-Identifier: MPL-2.0
                                                                                                       
provider "azurerm" {                                                       
  features {}                                                                       
}

variable "prefix" {
  description = "The prefix which should be used for all resources in this example"
}

variable "location" {
  description = "The Azure Region in which all resources in this example should be created."
}                                                                      
                                                                                    
resource "azurerm_resource_group" "main" {                                          
  name     = "${var.prefix}-resources"                                              
  location = var.location                                                           
}                                                                                   
                                                                                    
resource "azurerm_virtual_network" "main" {                                         
  name                = "${var.prefix}-network"                                     
  address_space       = ["10.0.0.0/16"]                                             
  location            = azurerm_resource_group.main.location                        
  resource_group_name = azurerm_resource_group.main.name                            
}                                                                                   
                                                                                    
resource "azurerm_subnet" "internal" {                                              
  name                 = "internal"                                                 
  resource_group_name  = azurerm_resource_group.main.name                           
  virtual_network_name = azurerm_virtual_network.main.name                          
  address_prefixes     = ["10.0.2.0/24"]                                            
}

resource "azurerm_network_interface" "main" {                                       
  name                = "${var.prefix}-nic"                                         
  resource_group_name = azurerm_resource_group.main.name                            
  location            = azurerm_resource_group.main.location                        
                                                                                    
  ip_configuration {                                                                
    name                          = "internal"                                      
    subnet_id                     = azurerm_subnet.internal.id                      
    private_ip_address_allocation = "Dynamic"                                       
  }                                                                                 
}                                                                                   
                                                                                    
resource "azurerm_linux_virtual_machine" "main" {                                   
  name                            = "${var.prefix}-vm"                              
  resource_group_name             = azurerm_resource_group.main.name                
  location                        = azurerm_resource_group.main.location            
  size                            = "Standard_F2s_v2"                               
  admin_username                  = "adminuser"                                     
  admin_password                  = "P@ssw0rd1234!"                                 
  disable_password_authentication = false                                           
  network_interface_ids = [                                                         
    azurerm_network_interface.main.id,                                              
  ]                                                                                 
                                                                                    
  source_image_reference {                                                          
    publisher = "Canonical"                                                         
    offer     = "0001-com-ubuntu-server-jammy"                                      
    sku       = "22_04-lts"                                                         
    version   = "latest"                                                            
  }

  os_disk {
## the following line is commented-out:                                                                      
#   storage_account_type = "Standard_LRS"                                           
    caching              = "ReadWrite"                                              
                                                                                    
    diff_disk_settings {                                                            
      option = "Local"                                                              
    }                                                                               
  }                                                                                 
}

Debug Output/Panic Output

│ Error: Missing required argument
│ 
│   on main.tf line 58, in resource "azurerm_linux_virtual_machine" "main":
│   58:   os_disk {
│ 
│ The argument "storage_account_type" is required, but no definition was found.

Expected Behaviour

Currently, when an ephemeral OS disk is defined for a VM or VM scale set, the os_disk storage_account_type property must still be defined even when an ephemeral OS disk is used (i.e., a diff_disk_settings block is defined in the os_disk block). In this case, storage_account_type is misleading at best, as there is no storage account backing an ephemeral OS disk as there (implicitly) is with a managed disk. Moreover, its presence could perhaps impart a false sense of security, as there is no data replication (e.g., LRS) with an ephemeral OS disk.

The example ARM template referenced by the azurerm docs for deploying a VM with an ephemeral OS disk omits the storageAccountType property:

terraform-provider-azurerm/website/docs/r/linux_virtual_machine.html.markdown

Line 303 in 7f80c0b

-> **NOTE:** `diff_disk_settings` can only be set when `caching` is set to `ReadOnly`. More information can be found [here](https://docs.microsoft.com/azure/virtual-machines/ephemeral-os-disks-deploy#vm-template-deployment)

If there is consensus for changing the current requirement that an storage_account_type assignment exist even for ephemeral OS disks, azurerm v3.x could make storage_account_type optional when diff_disk_settings is defined (for backwards compatibility), while v4.x could emit an error if storage_account_type is defined when diff_disk_settings is defined.

Actual Behaviour

No response

Steps to Reproduce

curl -LO https://raw.githubusercontent.com/hashicorp/terraform-provider-azurerm/v3.96.0/examples/virtual-machines/linux/ephemeral-os-disk/variables.tf
curl -L https://raw.githubusercontent.com/hashicorp/terraform-provider-azurerm/v3.96.0/examples/virtual-machines/linux/ephemeral-os-disk/main.tf | sed '/storage_account_type/s/^/#/' > main.tf
terraform init
terraform validate

### Important Factoids

_No response_

### References

_No response_