Configure host pool load balancing in Azure Virtual Desktop

Article
09/27/2024

Azure Virtual Desktop supports two load balancing algorithms for pooled host pools. Each algorithm determines which session host is used when a user starts a remote session. Load balancing doesn't apply to personal host pools because users always have a 1:1 mapping to a session host within the host pool.

The following load balancing algorithms are available for pooled host pools:

Breadth-first, which aims to evenly distribute new user sessions across the session hosts in a host pool. You don't have to specify a maximum session limit for the number of sessions.
Depth-first, which keeps starting new user sessions on one session host until the maximum session limit is reached. Once the session limit is reached, any new user connections are directed to the next session host in the host pool until it reaches its session limit, and so on.

You can only configure one of the load balancing algorithms at a time per pooled host pool, but you can change which one is used at any time. Both load balancing algorithms share the following behaviors:

If a user already has an active or disconnected session in the host pool and signs in again, the load balancer will successfully redirect them to the session host with their existing session. This behavior applies even if drain mode has been enabled for that session host.
If a user doesn't already have a session on a session host in the host pool, the load balancer doesn't consider a session host where drain mode has been enabled.
If you lower the maximum session limit on a session host while it has active user sessions, the change doesn't affect existing user sessions.

Breadth-first load balancing algorithm

The breadth-first load balancing algorithm aims to distribute user sessions across session hosts to optimize for session performance. Breadth-first is ideal for organizations that want to provide the best experience for users connecting to their remote resources as session host resources, such as CPU, memory, and disk, are generally less contended.

The breadth-first algorithm first queries session hosts in a host pool that allow new connections. The algorithm then selects a session host randomly from half the set of available session hosts with the fewest sessions. For example, if there are nine session hosts with 11, 12, 13, 14, 15, 16, 17, 18, and 19 sessions, a new session doesn't automatically go to the session host with the fewest sessions. Instead, it can go to any of the first five session hosts with the fewest sessions at random. Due to the randomization, some sessions may not be evenly distributed across all session hosts.

Depth-first load balancing algorithm

The depth-first load balancing algorithm aims to saturate one session host at a time. This algorithm is ideal for cost-conscious organizations that want more granular control on the number of session hosts available in a host pool, enabling you to more easily scale down the number of session hosts powered on when there are fewer users.

The depth-first algorithm first queries session hosts that allow new connections and haven't reached their maximum session limit. The algorithm then selects the session host with most sessions. If there's a tie, the algorithm selects the first session host from the query.

You must set a maximum session limit when using the depth-first algorithm. You can use Azure Virtual Desktop Insights to monitor the number of sessions on each session host and review session host performance to help determine the best maximum session limit for your environment.

Important

Once all session hosts have reached the maximum session limit, you need to increase the limit or add more session hosts to the host pool.

Prerequisites

To configure load balancing for a pooled host pool, you need:

An existing pooled host pool.
An Azure account assigned the Desktop Virtualization Host Pool Contributor role.
If you want to use Azure CLI or Azure PowerShell locally, see Use Azure CLI and Azure PowerShell with Azure Virtual Desktop to make sure you have the desktopvirtualization Azure CLI extension or the Az.DesktopVirtualization PowerShell module installed.

Configure load balancing

Select the relevant tab for your scenario.

Here's how to configure load balancing with the Azure portal:

Sign in to the Azure portal.
In the search bar, enter Azure Virtual Desktop and select the matching service entry
Select Host pools, then select the name of the host pool you want to configure.
Select Properties.
For Load balancing algorithm, select which type you want to use for this host pool from the drop-down menu, then for Max session limit, enter a value.
Select Save to apply the new load balancing settings.

Here's how to configure load balancing with Azure PowerShell. Be sure to change the <placeholder> values for your own.

Run PowerShell on your local device.
- If you're using PowerShell locally, first sign in with Azure PowerShell, and then make sure your Azure context is set to the subscription that you want to use.

Configure a host pool to perform breadth-first or depth-first load balancing using the Update-AzWvdHostPool cmdlet. Here are some examples:

To set breadth-first without adjusting the maximum session limit, run the following command:

$parameters = @{
    Name = '<HostPoolName>'
    ResourceGroupName = '<ResourceGroupName>'
    LoadBalancerType = 'BreadthFirst'
}

Update-AzWvdHostPool @parameters

To set depth-first and adjust the maximum session limit to 10, run the following command:

$parameters = @{
    Name = '<HostPoolName>'
    ResourceGroupName = '<ResourceGroupName>'
    LoadBalancerType = 'DepthFirst'
    MaxSessionLimit = '10'
}

Update-AzWvdHostPool @parameters

To make sure the setting has updated, run this command:

$parameters = @{
    Name = '<HostPoolName>'
    ResourceGroupName = '<ResourceGroupName>'
}

Get-AzWvdHostPool @parameters | Format-Table Name, LoadBalancerType, MaxSessionLimit

The output should be similar to the following example:

Name        LoadBalancerType MaxSessionLimit
----------- ---------------- ---------------
contosohp01 DepthFirst                    10

Here's how to configure load balancing with Azure CLI:

Run Azure CLI on your local device.
- If you're using the Azure CLI locally, first sign in with the Azure CLI, and then make sure your Azure context is set to the subscription that you want to use.

Configure a host pool to perform breadth-first or depth-first load balancing. Here are some examples:

To set breadth-first without adjusting the maximum session limit, run the following command:

az desktopvirtualization hostpool update \
    --resource-group <ResourceGroupName> \
    --name <HostPoolName> \
    --load-balancer-type BreadthFirst

To set depth-first and adjust the maximum session limit to 10, run the following command:

az desktopvirtualization hostpool update \
    --resource-group <ResourceGroupName> \
    --name <HostPoolName> \
    --load-balancer-type DepthFirst \
    --max-session-limit 10