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Using disks in Azure Resource Manager Templates

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Applies to: ✔️ Linux VMs ✔️ Windows VMs ✔️ Flexible scale sets ✔️ Uniform scale sets

This document walks through the differences between managed and unmanaged disks when using Azure Resource Manager templates to provision virtual machines. The examples help you to update existing templates that are using unmanaged Disks to managed disks. For reference, we are using the vm-simple-windows template as a guide. You can see the template using both managed Disks and a prior version using unmanaged disks if you'd like to directly compare them.

Unmanaged Disks template formatting

To begin, let's take a look at how unmanaged disks are deployed. When creating unmanaged disks, you need a storage account to hold the VHD files. You can create a new storage account or use one that already exists. This article shows you how to create a new storage account. Create a storage account resource in the resources block as shown below.

{
    "type": "Microsoft.Storage/storageAccounts",
    "apiVersion": "2018-07-01",
    "name": "[variables('storageAccountName')]",
    "location": "[resourceGroup().location]",
    "sku": {
        "name": "Standard_LRS"
    },
    "kind": "Storage",
    "properties": {}
}

Within the virtual machine object, add a dependency on the storage account to ensure that it's created before the virtual machine. Within the storageProfile section, specify the full URI of the VHD location, which references the storage account and is needed for the OS disk and any data disks.

{
    "type": "Microsoft.Compute/virtualMachines",
    "apiVersion": "2018-10-01",
    "name": "[variables('vmName')]",
    "location": "[resourceGroup().location]",
    "dependsOn": [
    "[resourceId('Microsoft.Storage/storageAccounts/', variables('storageAccountName'))]",
    "[resourceId('Microsoft.Network/networkInterfaces/', variables('nicName'))]"
    ],
    "properties": {
        "hardwareProfile": {...},
        "osProfile": {...},
        "storageProfile": {
            "imageReference": {
                "publisher": "MicrosoftWindowsServer",
                "offer": "WindowsServer",
                "sku": "[parameters('windowsOSVersion')]",
                "version": "latest"
            },
            "osDisk": {
                "name": "osdisk",
                "vhd": {
                    "uri": "[concat(reference(resourceId('Microsoft.Storage/storageAccounts/', variables('storageAccountName'))).primaryEndpoints.blob, 'vhds/osdisk.vhd')]"
                },
                "caching": "ReadWrite",
                "createOption": "FromImage"
            },
            "dataDisks": [
                {
                    "name": "datadisk1",
                    "diskSizeGB": 1023,
                    "lun": 0,
                    "vhd": {
                        "uri": "[concat(reference(resourceId('Microsoft.Storage/storageAccounts/', variables('storageAccountName'))).primaryEndpoints.blob, 'vhds/datadisk1.vhd')]"
                    },
                    "createOption": "Empty"
                }
            ]
        },
        "networkProfile": {...},
        "diagnosticsProfile": {...}
    }
}

Managed disks template formatting

With Azure Managed Disks, the disk becomes a top-level resource and no longer requires a storage account to be created by the user. Managed disks were first exposed in the 2016-04-30-preview API version, they are available in all subsequent API versions and are now the default disk type. The following sections walk through the default settings and detail how to further customize your disks.

Note

It is recommended to use an API version later than 2016-04-30-preview as there were breaking changes between 2016-04-30-preview and 2017-03-30.

Default managed disk settings

To create a VM with managed disks, you no longer need to create the storage account resource. Referencing the template example below, there are some differences from the previous unmanaged disk examples to note:

  • The apiVersion is a version that supports managed disks.
  • osDisk and dataDisks no longer refer to a specific URI for the VHD.
  • When deploying without specifying additional properties, the disk will use a storage type based on the size of the VM. For example, if you are using a VM size that supports premium storage (sizes with "s" in their name such as Standard_D2s_v3) then premium disks will be configured by default. You can change this by using the sku setting of the disk to specify a storage type.
  • If no name for the disk is specified, it takes the format of <VMName>_OsDisk_1_<randomstring> for the OS disk and <VMName>_disk<#>_<randomstring> for each data disk.
    • If a VM is being created from a custom image then the default settings for storage account type and disk name are retrieved from the disk properties defined in the custom image resource. These can be overridden by specifying values for these in the template.
  • By default, Azure disk encryption is disabled.
  • By default, disk caching is Read/Write for the OS disk and None for data disks.
  • In the example below there is still a storage account dependency, though this is only for storage of diagnostics and is not needed for disk storage.
{
    "type": "Microsoft.Compute/virtualMachines",
    "apiVersion": "2018-10-01",
    "name": "[variables('vmName')]",
    "location": "[resourceGroup().location]",
    "dependsOn": [
        "[resourceId('Microsoft.Storage/storageAccounts/', variables('storageAccountName'))]",
        "[resourceId('Microsoft.Network/networkInterfaces/', variables('nicName'))]"
    ],
    "properties": {
        "hardwareProfile": {...},
        "osProfile": {...},
        "storageProfile": {
            "imageReference": {
                "publisher": "MicrosoftWindowsServer",
                "offer": "WindowsServer",
                "sku": "[parameters('windowsOSVersion')]",
                "version": "latest"
            },
            "osDisk": {
                "createOption": "FromImage"
            },
            "dataDisks": [
                {
                    "diskSizeGB": 1023,
                    "lun": 0,
                    "createOption": "Empty"
                }
            ]
        },
        "networkProfile": {...},
        "diagnosticsProfile": {...}
    }
}

Using a top-level managed disk resource

As an alternative to specifying the disk configuration in the virtual machine object, you can create a top-level disk resource and attach it as part of the virtual machine creation. For example, you can create a disk resource as follows to use as a data disk.

{
    "type": "Microsoft.Compute/disks",
    "apiVersion": "2018-06-01",
    "name": "[concat(variables('vmName'),'-datadisk1')]",
    "location": "[resourceGroup().location]",
    "sku": {
        "name": "Standard_LRS"
    },
    "properties": {
        "creationData": {
            "createOption": "Empty"
        },
        "diskSizeGB": 1023
    }
}

Within the VM object, reference the disk object to be attached. Specifying the resource ID of the managed disk created in the managedDisk property allows the attachment of the disk as the VM is created. The apiVersion for the VM resource is set to 2017-03-30. A dependency on the disk resource is added to ensure it's successfully created before VM creation.

{
    "type": "Microsoft.Compute/virtualMachines",
    "apiVersion": "2018-10-01",
    "name": "[variables('vmName')]",
    "location": "[resourceGroup().location]",
    "dependsOn": [
        "[resourceId('Microsoft.Storage/storageAccounts/', variables('storageAccountName'))]",
        "[resourceId('Microsoft.Network/networkInterfaces/', variables('nicName'))]",
        "[resourceId('Microsoft.Compute/disks/', concat(variables('vmName'),'-datadisk1'))]"
    ],
    "properties": {
        "hardwareProfile": {...},
        "osProfile": {...},
        "storageProfile": {
            "imageReference": {
                "publisher": "MicrosoftWindowsServer",
                "offer": "WindowsServer",
                "sku": "[parameters('windowsOSVersion')]",
                "version": "latest"
            },
            "osDisk": {
                "createOption": "FromImage"
            },
            "dataDisks": [
                {
                    "lun": 0,
                    "name": "[concat(variables('vmName'),'-datadisk1')]",
                    "createOption": "attach",
                    "managedDisk": {
                        "id": "[resourceId('Microsoft.Compute/disks/', concat(variables('vmName'),'-datadisk1'))]"
                    }
                }
            ]
        },
        "networkProfile": {...},
        "diagnosticsProfile": {...}
    }
}

Create managed availability sets with VMs using managed disks

To create managed availability sets with VMs using managed disks, add the sku object to the availability set resource and set the name property to Aligned. This property ensures that the disks for each VM are sufficiently isolated from each other to avoid single points of failure. Also note that the apiVersion for the availability set resource is set to 2018-10-01.

{
    "type": "Microsoft.Compute/availabilitySets",
    "apiVersion": "2018-10-01",
    "location": "[resourceGroup().location]",
    "name": "[variables('avSetName')]",
    "properties": {
        "PlatformUpdateDomainCount": 3,
        "PlatformFaultDomainCount": 2
    },
    "sku": {
        "name": "Aligned"
    }
}

Standard SSD disks

Below are the parameters needed in the Resource Manager template to create Standard SSD Disks:

  • apiVersion for Microsoft.Compute must be set as 2018-04-01 (or later)
  • Specify managedDisk.storageAccountType as StandardSSD_LRS

The following example shows the properties.storageProfile.osDisk section for a VM that uses Standard SSD Disks:

"osDisk": {
    "osType": "Windows",
    "name": "myOsDisk",
    "caching": "ReadWrite",
    "createOption": "FromImage",
    "managedDisk": {
        "storageAccountType": "StandardSSD_LRS"
    }
}

For a complete template example of how to create a Standard SSD disk with a template, see Create a VM from a Windows Image with Standard SSD Data Disks.

Additional scenarios and customizations

To find full information on the REST API specifications, please review the create a managed disk REST API documentation. You will find additional scenarios, as well as default and acceptable values that can be submitted to the API through template deployments.

Next steps