Support OS disk size or storage type changes

[r7vme]

Related issue: #345

This change allows to change OS disk size and/or storage type by forcing VM recreation.

This is my first code contribution to terraform provider. Please point me if docs/tests adjustments are needed.

Use case

This fits our use case: We switch our "ephemeral" VMs to new storage_type and want terraform to handle it. In current state it fails with

* azurerm_virtual_machine.test: compute.VirtualMachinesClient#CreateOrUpdate: Failure sending request: StatusCode=409 -- Original Error: failed r
equest: autorest/azure: Service returned an error. Status=<nil> Code="OperationNotAllowed" Message="Managed disk storage account type change thro
ugh Virtual Machine 'acctvm' is not allowed. Please update disk resource at /subscriptions/1be3b2e6-497b-45b9-915f-eb35cae23c6a/resourceGroups/ro
mantest1/providers/Microsoft.Compute/disks/myosdisk1." Target="osDisk.managedDisk.storageAccountType"

Limitations

This does not work if "delete_os_disk_on_termination" is set to "false" (default) and fails with

* azurerm_virtual_machine.test: Long running operation terminated with status 'Failed': Code="ConflictingUserInput" Message="Disk myosdisk1 alrea
dy exists in resource group ROMANTEST1. Only CreateOption.Attach is supported."  

What is the best way to handle such use cases?

Notes

This is nothing to do with data disks changes. In case data disks, i assume we need to gently detach disk/stop vm and make changes. #1207 looks like requirement for data disk changes.

[katbyte]

Hi @r7vme,

Thank you for opening this PR, this has been a long outstanding issue. Unfortunately it is not as easy as marking the properties as force new. This is because now when any additional disks are then added the resource will be recreated.

To correctly work around this a CustomizeDiff function needs to be added to only force a new resource on a diskrename/resize, while not doing a force new of the resource.

There is an old branch exploring this for disk renaming here (size would be the same in theory)

Happy to provide further guidance if you have any questions.

[JunyiYi]

Hi @r7vme , Thanks for the contribution. Besides the suggestions provided by @katbyte , you also need to update the document here: https://github.com/terraform-providers/terraform-provider-azurerm/blame/master/website/docs/r/virtual_machine.html.markdown#L297.

[r7vme]

thanks for suggestions, unfortunately i don't have timeslot now to work on this.

[ghost]

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