Attach or detach a Virtual Machine to or from a Virtual Machine Scale Set

Attaching a Virtual Machine to a Virtual Machine Scale Set

There are times where you need to attach a virtual machine (VM) to a Virtual Machine Scale Set to benefit from the scale, availability, and flexibility that comes with scale sets. There are two ways to attach VMs to scale sets: manually create a new standalone VM in the scale set or attach an existing VM to the scale set.

You can attach a new standalone VM to a scale set when you need a different configuration on a specific VM than what's defined in the scaling profile, or when the scale set doesn't have a scaling profile. Manually attaching VMs gives you full control over instance naming and placement into a specific availability zone or fault domain. The VM doesn't have to match the configuration in the scale set's scaling profile, so you can specify parameters like operating system, networking configuration, on-demand or Spot, and VM size.

You can attach an existing VM to an existing Virtual Machine Scale Set by specifying which scale set you would like to attach to. The VM doesn't have to be the same as the VMs already running in the scale set, meaning it can have a different operating system, network configuration, priority, disk, and more.

Attach a new Virtual Machine to a Virtual Machine Scale Set

Attach a virtual machine to a Virtual Machine Scale Set at the time of VM creation by specifying the virtualMachineScaleSet property.

az vm create 
  --name myVM \
  --resource-group myResourceGroup \
  --image Ubuntu2204 \
  --vmss myVMScaleSet \
  --platform-fault-domain 1

New-AzVm `
    -ResourceGroupName 'myResourceGroup' `
    -Name 'myVM' `
    -Location 'China North 2' `
    -VmssId 'myVmss' `
    -VirtualNetworkName 'myVnet' `
    -SubnetName 'mySubnet' `
    -SecurityGroupName 'myNetworkSecurityGroup' `
    -PublicIpAddressName 'myPublicIpAddress' `
    -OpenPorts 80,3389

Exceptions to attaching a new Virtual Machine to a Virtual Machine Scale Set

• The VM must be in the same resource group as the scale set.
• Regional virtual machines (no availability zones specified) can be attached to regional scale sets.
• Zonal virtual machines can be attached to scale sets that specify one or more zone. The virtual machine must be in one of the zones spanned by the scale set. For example, you can't create a virtual machine in Zone 1, and place it in a scale set that spans Zones 2 and 3.
• The scale set must be in Flexible orchestration mode, and the singlePlacementGroup property must be false.

Attach an existing Virtual Machine to a Virtual Machine Scale Set

Attach an existing virtual machine to a Virtual Machine Scale Set after the time of VM creation by specifying the virtualMachineScaleSet property. Attaching an existing VM to a scale set with a fault domain count of 1 doesn't require downtime.

az vm update 
  --resource-group {resourceGroupName} \
  --name {vmName} \
  --set virtualMachineScaleSet.id='/subscriptions/{subscriptionID}/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachineScaleSets/{scaleSetName}'

#Get VM information
$vm = Get-AzVM -ResourceGroupName $resourceGroupName -Name $vmName `

#Get scale set information
$vmss = Get-AzVmss -ResourceGroupName $resourceGroupName -Name $vmssName `

#Update the VM with the scale set ID
Update-AzVM -ResourceGroupName $resourceGroupName -VM $vm  -VirtualMachineScaleSetId $vmss.Id

Limitations for attaching an existing Virtual Machine to a scale set

• The scale set must use Flexible orchestration mode.
• The scale set must have a platformFaultDomainCount of 1.
• The VM and scale set must be in the same resource group.
• The VM and target scale set must both be zonal, or they must both be regional. You can't attach a zonal VM to a regional scale set.
• The VM can't be in a self-defined availability set.
• The VM can't be in a ProximityPlacementGroup.
• The VM can't be in an Azure Dedicated Host.
• The VM must have a managed disk.
• The scale set must have singlePlacementGroup set to False.
• Scale sets created without a scaling profile default to singlePlacementGroup set to null. To attach VMs to a scale set without a scaling profile, singlePlacementGroup needs to be set to False at the time of the scale set's creation.
• The VM can't be a Remote Direct Memory Access (RDMA) capable HB-series or N-series VM.

Detaching a Virtual Machine from a Virtual Machine Scale Set

Should you need to detach a VM from a scale set, you can follow the below steps to remove the VM from the scale set.

az vm update 
  --resource-group resourceGroupName \
  --name vmName \
  --set virtualMachineScaleSet.id=null 

#Get VM information
$vm = Get-AzVM -ResourceGroupName $resourceGroupName -Name $vmName 

#Update the VM with the new scale set refence of $null
Update-AzVM -ResourceGroupName $resourceGroupName -VM $vm -VirtualMachineScaleSetId $null

Limitations for detaching a Virtual Machine from a scale set

• The scale set must use Flexible orchestration mode.
• The scale set must have a platformFaultDomainCount of 1.
• Scale sets created without a scaling profile default to singlePlacementGroup set to null. To detach VMs from a scale set without a scaling profile, singlePlacementGroup needs to be set to False.
• The VM can't be an RDMA capable HB-series or N-series VM.

Moving Virtual Machines between scale sets

To move a VM from one scale set to another, use the following steps:

1. Detach the VM from scale set A.
2. Once the detach completes, attach the VM to scale set B.

Limitations

The limitations for VMs to be attached or detached to or from a scale set remain the same.

Troubleshooting

Attach an existing Virtual Machine to an existing scale set troubleshooting

Detach a Virtual Machine from a scale set troubleshooting

What's next

Learn how to manage updates and maintenance using Maintenance notification, Maintenance configurations, and Scheduled Events.