Scale session hosts dynamically

For many Windows Virtual Desktop deployments in Azure, the virtual machine costs represent significant portion of the total Windows Virtual Desktop deployment cost. To reduce costs, it's best to shut down and deallocate session host virtual machines (VMs) during off-peak usage hours, then restart them during peak usage hours.

This article uses a simple scaling script to automatically scale session host virtual machines in your Windows Virtual Desktop environment. To learn more about how the scaling script works, see the How the scaling script works section.

Prerequisites

The environment where you run the script must have the following things:

  • A Windows Virtual Desktop tenant and account or a service principal with permissions to query that tenant (such as RDS Contributor).
  • Session host pool VMs configured and registered with the Windows Virtual Desktop service.
  • An additional virtual machine that runs the scheduled task via Task Scheduler and has network access to session hosts. This will be referred to later in the document as scaler VM.
  • The Microsoft Azure Resource Manager PowerShell module installed on the VM running the scheduled task.
  • The Windows Virtual Desktop PowerShell module installed on the VM running the scheduled task.

Recommendations and limitations

When running the scaling script, keep the following things in mind:

  • This scaling script can only handle one host pool per instance of the scheduled task that is running the scaling script.
  • The scheduled tasks that run scaling scripts must be on a VM that is always on.
  • Create a separate folder for each instance of the scaling script and its configuration.
  • This script doesn't support signing in as an admin to Windows Virtual Desktop with Azure AD user accounts that require multi-factor authentication. We recommend you use service principals to access the Windows Virtual Desktop service and Azure. Follow this tutorial to create a service principal and a role assignment with PowerShell.
  • Azure's SLA guarantee applies only to VMs in an availability set. The current version of the document describes an environment with a single VM doing the scaling, which may not meet availability requirements.

Deploy the scaling script

The following procedures will tell you how to deploy the scaling script.

Prepare your environment for the scaling script

First, prepare your environment for the scaling script:

  1. Sign in to the VM (scaler VM) that will run the scheduled task with a domain administrative account.
  2. Create a folder on the scaler VM to hold the scaling script and its configuration (for example, C:\scaling-HostPool1).
  3. Download the basicScale.ps1, Config.xml, and Functions-PSStoredCredentials.ps1 files, and the PowershellModules folder from the scaling script repository and copy them to the folder you created in step 2. There are two primary ways to obtain the files before copying them to the scaler VM:
    • Clone the git repository to your local machine.
    • View the Raw version of each file, copy and paste the content of each file to a text editor, then save the files with the corresponding file name and file type.

Create securely stored credentials

Next, you'll need to create the securely stored credentials:

  1. Open PowerShell ISE as an administrator.

  2. Import the RDS PowerShell module by running the following cmdlet:

    Install-Module Microsoft.RdInfra.RdPowershell
    
  3. Open the edit pane and load the Function-PSStoredCredentials.ps1 file, then run the whole script (F5)

  4. Run the following cmdlet:

    Set-Variable -Name KeyPath -Scope Global -Value <LocalScalingScriptFolder>
    

    For example, Set-Variable -Name KeyPath -Scope Global -Value "c:\scaling-HostPool1"

  5. Run the New-StoredCredential -KeyPath $KeyPath cmdlet. When prompted, enter your Windows Virtual Desktop credentials with permissions to query the host pool (the host pool is specified in the config.xml).

    • If you use different service principals or standard account, run the New-StoredCredential -KeyPath $KeyPath cmdlet once for each account to create local stored credentials.
  6. Run Get-StoredCredential -List to confirm the credentials were created successfully.

Configure the config.xml file

Enter the relevant values into the following fields to update the scaling script settings in config.xml:

Field Description
AADTenantId Azure AD Tenant ID that associates the subscription where the session host VMs run
AADApplicationId Service principal application ID
AADServicePrincipalSecret This can be entered during the testing phase but is to be kept empty once you create credentials with Functions-PSStoredCredentials.ps1
currentAzureSubscriptionId The ID of the Azure subscription where the session host VMs run
tenantName Windows Virtual Desktop tenant name
hostPoolName Windows Virtual Desktop host pool name
RDBroker URL to the WVD service, default value https://rdbroker.wvd.microsoft.com
Username The service principal application ID (it’s possible to have the same service principal as in AADApplicationId) or standard user without multi-factor authentication
isServicePrincipal Accepted values are true or false. Indicates whether the second set of credentials being used is a service principal or a standard account.
BeginPeakTime When peak usage time begins
EndPeakTime When peak usage time ends
TimeDifferenceInHours Time difference between local time and UTC, in hours
SessionThresholdPerCPU Maximum number of sessions per CPU threshold used to determine when a new session host VM needs to be started during peak hours.
MinimumNumberOfRDSH Minimum number of host pool VMs to keep running during off-peak usage time
LimitSecondsToForceLogOffUser Number of seconds to wait before forcing users to sign out. If set to 0, users aren’t forced to sign out.
LogOffMessageTitle Title of the message sent to a user before they’re forced to sign out
LogOffMessageBody Body of the warning message sent to users before they’re signed out. For example, "This machine will shut down in X minutes. Please save your work and sign out.”

Configure the Task Scheduler

After configuring the configuration .xml file, you'll need to configure the Task Scheduler to run the basicScaler.ps1 file at a regular interval.

  1. Start Task Scheduler.
  2. In the Task Scheduler window, select Create Task…
  3. In the Create Task dialog, select the General tab, enter a Name (for example, “Dynamic RDSH”), select Run whether user is logged on or not and Run with highest privileges.
  4. Go to the Triggers tab, then select New…
  5. In the New Trigger dialog, under Advanced settings, check Repeat task every and select the appropriate period and duration (for example, 15 minutes or Indefinitely).
  6. Select the Actions tab and New…
  7. In the New Action dialog, enter powershell.exe into the Program/script field, then enter C:\scaling\basicScale.ps1 into the Add arguments (optional) field.
  8. Go to the Conditions and Settings tabs and select OK to accept the default settings for each.
  9. Enter the password for the administrative account where you plan to run the scaling script.

How the scaling script works

This scaling script reads settings from a config.xml file, including the start and end of the peak usage period during the day.

During peak usage time, the script checks the current number of sessions and the current running RDSH capacity for each host pool. It calculates if the running session host VMs have enough capacity to support existing sessions based on the SessionThresholdPerCPU parameter defined in the config.xml file. If not, the script starts additional session host VMs in the host pool.

During the off-peak usage time, the script determines which session host VMs should shut down based on the MinimumNumberOfRDSH parameter in the config.xml file. The script will set the session host VMs to drain mode to prevent new sessions connecting to the hosts. If you set the LimitSecondsToForceLogOffUser parameter in the config.xml file to a non-zero positive value, the script will notify any currently signed in users to save work, wait the configured amount of time, and then force the users to sign out. Once all user sessions have been signed off on a session host VM, the script will shut down the server.

If you set the LimitSecondsToForceLogOffUser parameter in the config.xml file to zero, the script will allow the session configuration setting in the host pool properties to handle signing off user sessions. If there are any sessions on a session host VM, it will leave the session host VM running. If there aren't any sessions, the script will shut down the session host VM.

The script is designed to run periodically on the scaler VM server using Task Scheduler. Select the appropriate time interval based on the size of your Remote Desktop Services environment, and remember that starting and shutting down virtual machines can take some time. We recommend running the scaling script every 15 minutes.

Log files

The scaling script creates two log files, WVDTenantScale.log and WVDTenantUsage.log. The WVDTenantScale.log file will log the events and errors (if any) during each execution of the scaling script.

The WVDTenantUsage.log file will record the active number of cores and active number of virtual machines each time you execute the scaling script. You can use this information to estimate the actual usage of Microsoft Azure VMs and the cost. The file is formatted as comma-separated values, with each item containing the following information:

time, host pool, cores, VMs

The file name can also be modified to have a .csv extension, loaded into Microsoft Excel, and analyzed.