Manage host pools (RD Session Hosts)

When you publish resources in Parallels RAS, you need to specify one or more servers that host them. Host pools allow you to combine multiple RD Session Hosts and then publish the resources from the host pool instead of specifying individual servers.

The main benefits of using RD Session Host host pools are as follows:

  • They simplify the management of published resources.

  • They allow you to use RD Session Hosts created from a template. More on this later in this section.

Each RD Session Hosts must belong to a host pool. Parallels RAS comes with a built-in host pool named <Default> that you can use. Note that an RD Session Host can be a member of one host pool only. You cannot add the same server to multiple host pools.

Moving an RD Session Host to another host pool

To move an RD Session Host from one host pool to another:

  1. In the RAS console, navigate to Farm > <Site> > RD Session Hosts.

  2. Select an RD Session Host.

  3. Click Tasks > Assign to host pool or right-click the RD Session Host and select Assign to host pool in the context menu.

  4. In the Assign to Host pool dialog, select the host pool you need.

Note: The settings of the new host pool will apply to the RD Session Host.


The settings on the Autoscale tab of the host pool properties determine how RD Session Hosts are created from the specified template. The settings are described below.

Template: Specifies the template assigned to the host pool.

Enable autoscale: Enables autoscale.

Configure: Configures the autoscale settings:

  • Min number of hosts to be added to the host pool from the Template: Specifies the minimum number of servers that will be added to the host pool automatically when the template is assigned to the host pool. This number of servers will remain in the host pool irrespective of utilization.

  • Max number of hosts to be added to the host pool from the Template: This option allows you to set a limit on how many servers in total can be added to the host pool from the template. A template can be shared between host pools. By setting a limit for each host pool, you can ensure that the combined number of servers in each host pool will not exceed the template limit. Consider the following examples:

    • If the template is used by a single host pool, then this number can be up to the Maximum hosts setting of the template.

    • If two or more host pools share the same template, then the combined number from all host pools must be less or equal to the Maximum hosts settings of the template.

    When you save a host pool, a validation will be performed against other host pools (if any) and you will see an error message if the numbers don't match. Note that when a server cannot be created on request due to an error, a "Template error" event is triggered and the administrator will receive an alert message.

  • Add new or power on existing hosts when workload is above (%): Specifies a workload threshold in percent. When the actual workload is above this value, a new server (or servers) will be created and added to the host pool (if not already available). The host pool workload percentage is calculated using the following formula:

    Host pool Workload = (Current Sessions / Max Sessions) * 100

    In the formula above:

    • Current Sessions is the total number of all sessions on all servers in the host pool. This includes static (standalone) servers and servers created from the template (host pools). Note that servers that are disabled, being drained, or have the agent status of ‘Not Verified’ are not included in the calculation.

    • Max Sessions is a setting that you specify on the Agent Settings tab (either inherited from Site defaults or overridden for this host pool) and the maximum number of sessions allowed for the host pool.

    Consider the following examples:

    RAS Host pool 1 — mixed server types (static and host pools), different agent status:

    • RDSH-1, Status: OK, Max Sessions 10, Current Sessions: 2, Type: Static

    • RDSH-2, Status: Disabled, Max Sessions 20, Current Sessions: 0, Type: Static

    • RDSH-3, Status: OK, Max sessions 10, Current Sessions: 4, Type: Host

    • RDSH-4, Status: Drain Mode, Max sessions 10, Current Sessions: 3, Type: Host

    For the host pool above, the workload is calculated as (Current Sessions / Max Sessions) * 100 or ((2 + 4) / 20) * 100 = 30%

    Note that servers RDSH-2 and RDSH-4 are not included in the workload because the former has the agent disabled and the latter is in drain mode.

    RAS Host pool 2 — mixed server types (static and host pools), different agent status:

    • RDSH-1, Status: OK, Max Sessions 10, Current Sessions: 0, Type: Host

    • RDSH-2, Status: OK, Max Sessions 10, Current Sessions: 2, Type: Host

    • RDSH-3, Status: Not Verified, Max sessions 10, Current Sessions: 0, Type: Host

    Host pool Workload = (Current Sessions / Max Sessions) * 100 or ((0 + 2) / 20) * 100 = 10%

    Please note that a host pool will always make sure that it has at least one server available, even if the workload is zero percent.

  • Number of hosts to be added to the host pool per request: Specifies how many servers should be created when the workload goes above the threshold value. This setting works together with the Add servers from template when workload is above (%) setting described above. When a host pool sends a request to the template to create additional servers, the value specified here will determine the number of servers that will be created.

  • Drain and power off hosts from host pool when workload is below (%): Specifies a workload threshold in percent. When the actual workload is below this value and remains there for a period specified in the Workload remains below this level field, excessive hosts will be switched to drain mode or powered off. The period of time can be selected from the drop-down list or you can type your own integer value using "weeks", "days", "hours", "minutes", or "seconds" as a unit measure. The server(s) with the least number of sessions will be switched to drain mode. As soon as all users are logged off from a server, it is unassigned from the host pool. At that point, the server becomes available to other host pools on demand.

  • Remove hosts from host pool after drain and power off: Specifies if hosts should be removed from the host pool after being drained and powered off.

Tip: Servers are unassigned from the host pool only when all user sessions on that particular server are logged off. In case user sessions are still present, such as user sessions in idle, active or disconnected state, autoscaling does not log off user sessions and does not unassign the server from a host pool.

Note: Parallels recommends setting viable timeouts for idle time and disconnected sessions either in Windows Host pool Policies or in the Site Default Properties dialog to make the drain mode effective. GPOs can be used to forcibly log off a user session, however this should be used carefully as this may result in data loss.

Using host pool defaults

RD Sessions Hosts assigned to a host pool have various settings that they can inherit from the host pool defaults. This makes it simpler to configure a single set of settings for all servers instead of configuring each server individually. A Site also has its own default settings (Site defaults). Moreover, an RD Session Host host pool can inherit these Site defaults. This gives you the following choices when inheriting default settings by an RD Session Host:

  • Configure Site defaults and make the host pool inherit these settings. The RD Session Hosts assigned to the host pool will therefore also inherit Site defaults. This is the default scenario for a new host pool. Site defaults can be configured by navigating to Farm > <Site> > RD Session hosts and clicking Tasks > Site defaults.

  • Configure default settings for a given host pool. This way you can have multiple host pools, each having its own host pool defaults (different from Site defaults). Therefore, the servers assigned to a host pool will inherit the host pool's defaults.

To configure default settings for a host pool, open the Host pool Properties dialog (Tasks > Properties), select a desired tab (except the General tab, which doesn't have any defaults) and select or clear the Inherit default settings option. If you clear the option, you can specify your own defaults. All servers that are (or will be) assigned to this host pool will inherit these settings. Note that inheritance works independently for each individual tab on the host pool properties dialog.

For information on how default settings are configured for an RD Session Host, see View and modify RD Session Host properties.

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