Understand names and instance IDs for Azure Virtual Machine Scale Set VMs

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08/22/2024

Each VM in a scale set gets a name and instance ID that uniquely identifies it. These are used in the scale set APIs to do operations on a specific VM in the scale set. This article describes instance IDs for scale sets and the various ways they surface.

Scale set VM names

Virtual Machine Scale Sets will generate a unique name for each VM in the scale set. This name is used as part of the unique resource identifier for Azure ARM resources. The maximum length of the instance resource name is 64 characters. The naming convention differs by orchestration mode:

Flexible orchestration mode instance name: {scale-set-name}_{8-char-guid}
Uniform orchestration mode instance name: {scale-set-name}_{instance-id}

Scale set instance ID for Flexible Orchestration Mode

For Virtual Machine Scale Sets in Flexible Orchestration mode, the instance ID is simply the name of the virtual machine.

Scale set instance ID for Uniform Orchestration Mode

For scale sets in Uniform orchestration mode, the instance ID a decimal number. The instance IDs may be reused for new instances once old instances are deleted.

Note

There is no guarantee on the way instance IDs are assigned to the VMs in the scale set. They might seem sequentially increasing at times, but this is not always the case. Do not take a dependency on the specific way in which instance IDs are assigned to the VMs.

You can get the list of instance IDs by listing all instances in a scale set.

REST API

For more information, see the REST API documentation.

GET https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachineScaleSets/{vmScaleSetName}/virtualMachines?api-version={apiVersion}

You can also specify a specific instance ID to reimage when using the reimage API. For more information, see the REST API documentation

POST https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachineScaleSets/{vmScaleSetName}/virtualmachines/{instanceId}/reimage?api-version={apiVersion}

PowerShell

For more information, see the PowerShell documentation.

Get-AzVmssVM -ResourceGroupName {resourceGroupName} -VMScaleSetName {vmScaleSetName}

You can also specify a specific instance ID to reimage when using the reimage API. For more information, see the PowerShell documentation

Set-AzVmssVM -ResourceGroupName {resourceGroupName} -VMScaleSetName {vmScaleSetName} -InstanceId {instanceId} -Reimage

CLI

For more information, see the CLI documentation.

az vmss list-instances -g {resourceGroupName} -n {vmScaleSetName}

You can also specify a specific instance ID to reimage when using the reimage API. For more information, see the CLI documentation.

az vmss reimage -g {resourceGroupName} -n {vmScaleSetName} --instance-id {instanceId}

Instance Metadata VM name

If you query the instance metadata from within a scale set VM, you see a "name" in the output:

{
  "compute": {
    "location": "westus",
    "name": "nsgvmss_85",

Scale set VM computer name

Each VM in a scale set also gets a computer name assigned to it. This computer name is the hostname of the VM in the Azure-provided DNS name resolution within the virtual network. The computer name naming convention differs by orchestration mode:

Flexible orchestration mode: {computer-name-prefix}{6-char-guid}
Uniform orchestration mode: {computer-name-prefix}{base-36-instance-id}

The computer name prefix is a property of the scale set model that you can set, so it can be different from the scale set name itself. The scale set VM computer name can also be changed from inside the guest OS once the VM has been created. The maximum length of the computer name is operating system dependent. For more information, see Naming rules and restrictions..

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