How to expand the OS drive of a Virtual Machine in an Azure Resource Group

Overview

When you create a new virtual machine (VM) in a Resource Group by deploying an image from Azure Marketplace, the default OS drive is often 127 GB (some images have smaller OS disk sizes by default). Even though it’s possible to add data disks to the VM (how many depending upon the SKU you’ve chosen) and moreover it’s recommended to install applications and CPU intensive workloads on these addendum disks, oftentimes customers need to expand the OS drive to support certain scenarios such as following:

1. Support legacy applications that install components on OS drive.
2. Migrate a physical PC or virtual machine from on-premises with a larger OS drive.

Resize the OS drive

In this article we’ll accomplish the task of resizing the OS drive using resource manager modules of Azure Powershell. Open your Powershell ISE or Powershell window in administrative mode and follow the steps below:

1. Sign-in to your Microsoft Azure account in resource management mode and select your subscription as follows:

   Login-AzureRmAccount
   Select-AzureRmSubscription –SubscriptionName 'my-subscription-name'
   

2. Set your resource group name and VM name as follows:

   $rgName = 'my-resource-group-name'
   $vmName = 'my-vm-name'
   

3. Obtain a reference to your VM as follows:

   $vm = Get-AzureRmVM -ResourceGroupName $rgName -Name $vmName
   

4. Stop the VM before resizing the disk as follows:

   Stop-AzureRmVM -ResourceGroupName $rgName -Name $vmName
   

5. And here comes the moment we’ve been waiting for! Set the size of the OS disk to the desired value and update the VM as follows:

   $vm.StorageProfile.OSDisk.DiskSizeGB = 1023
   Update-AzureRmVM -ResourceGroupName $rgName -VM $vm
   

   Warning

   The new size should be greater than the existing disk size. The maximum allowed is 2048 GB. (It is possible to expand the VHD blob beyond that size, but the OS will only be able to work with the first 2048 GB of space.)

6. Updating the VM may take a few seconds. Once the command finishes executing, restart the VM as follows:

   Start-AzureRmVM -ResourceGroupName $rgName -Name $vmName
   

And that’s it! Now RDP into the VM, open Computer Management (or Disk Management) and expand the drive using the newly allocated space.

Summary

In this article, we used Azure Resource Manager modules of Powershell to expand the OS drive of an IaaS virtual machine. Reproduced below is the complete script for your reference:

Login-AzureRmAccount
Select-AzureRmSubscription -SubscriptionName 'my-subscription-name'
$rgName = 'my-resource-group-name'
$vmName = 'my-vm-name'
$vm = Get-AzureRmVM -ResourceGroupName $rgName -Name $vmName
Stop-AzureRmVM -ResourceGroupName $rgName -Name $vmName
$vm.StorageProfile.OSDisk.DiskSizeGB = 1023
Update-AzureRmVM -ResourceGroupName $rgName -VM $vm
Start-AzureRmVM -ResourceGroupName $rgName -Name $vmName

Next Steps

Though in this article, we focused primarily on expanding the OS disk of the VM, the developed script may also be used for expanding the data disks attached to the VM by changing a single line of code. For example, to expand the first data disk attached to the VM, replace the OSDisk object of StorageProfile with DataDisks array and use a numeric index to obtain a reference to first attached data disk, as shown below:

$vm.StorageProfile.DataDisks[0].DiskSizeGB = 1023

Similarly you may reference other data disks attached to the VM, either by using an index as shown above or the Name property of the disk as illustrated below:

($vm.StorageProfile.DataDisks | Where {$_.Name -eq 'my-second-data-disk'})[0].DiskSizeGB = 1023

If you want to find out how to attach disks to an Azure Resource Manager VM, check this article.