Optimizing and Managing Power Automatically

You can configure Workload Balancing to accept optimization recommendations automatically (Automation) and turn servers on or off automatically (Power Management).

Accepting Optimization Recommendations automatically

Workload Balancing lets you configure for it to accept optimization recommendations on your behalf and perform the optimization actions it recommends automatically. You can use this feature, which is known as Automation , to apply any recommendations automatically, including ones to improve performance or power down hosts. However, to power down hosts as virtual-machines usage drops, you must configure automation, power management, and Maximum Density mode.

By default, Workload Balancing does not accept optimizations automatically. You must enable Automation if you want Workload Balancing to accept recommendations automatically. If you do not, Workload Balancing still prompts you to accept recommendations manually.

Workload Balancing will not automatically apply recommendations to hosts or virtual machines if the recommendations conflict with High Availability settings. If a pool will become overcommitted by applying Workload Balancing optimization recommendations, XenCenter will prompt you whether or not you want to continue applying the recommendation. When Automation is enabled, Workload Balancing will not apply any power-management recommendations that exceed the number of host failures to tolerate in the High Availability plan.

It is possible to tweak how Workload Balancing applies recommendations in automated mode. For information, see Advanced Settings.

Enabling Power Management

The term power management refers to the ability to the turn the power on or off for physical hosts. In a Workload Balancing context, this term refers to powering hosts in a pool on or off based on the pool’s total workload.

Configuring Workload Balancing power management on a host requires that:

  • The hardware for the host server has remote power on/off capabilities
  • The Host Power On feature is configured for the host
  • The host has been explicitly selected as a host to participate in (Workload Balancing) Power Management

In addition, if you want Workload Balancing to power off hosts automatically, you also need to configure:

  • Workload Balancing is configured to apply recommendations automatically
  • Workload Balancing is configured to apply Power Management recommendations automatically

When enabled and the pool is in Maximum Density mode, if Workload Balancing detects unused resources, it recommends powering off hosts until it eliminates all excess capacity in the pool. If Workload Balancing detects there is not sufficient host capacity in the pool to turn off servers, it recommends leaving the servers on until the pool’s workload decreases enough to power off servers. When you configure Workload Balancing to power off extra servers automatically, it applies these recommendations automatically and, consequently, behaves in the same way.

When a host is set to participate in Power Management, Workload Balancing makes power-on/off recommendations as needed. If you turn on the option to apply Power Management recommendations automatically, you do so at the pool level. However, you can specify which hosts from the pool you want to participate in Power Management.

Understanding Power Management Behavior

Before Workload Balancing recommends powering hosts on or off, it selects the hosts to transfer virtual machines to (that is, to “fill”). It does so in the following order:

  1. Filling the pool master since it is the host that cannot be powered off.
  2. Filling the host with the most virtual machines.
  3. Filling subsequent hosts according to which hosts have the most virtual machines running.

When Workload Balancing fills the pool master, it does so assuming artificially low (internal) thresholds for the master. Workload Balancing uses these low thresholds as a buffer to prevent the pool master from being overloaded.

Workload Balancing fills hosts in this order to encourage density.

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This illustration shows how, when consolidating VMs on hosts in Maximum Density mode, XenServer attempts to fill the pool master first, the most loaded server second, and the least loaded server third.

If Workload Balancing detects a performance issue while the pool is in Maximum Density mode, it attempts to address the issue by recommending migrating workloads among the powered-on hosts. If Workload Balancing cannot resolve the issue using this method, it attempts to power-on a host. (Workload Balancing determines which host(s) to power-on by applying the same criteria it would if the optimization mode was set to Maximum Performance.)

When Workload Balancing is running in Maximum Performance mode, Workload Balancing recommends powering on hosts until the resource utilization on all hosts in the pool falls below the High threshold.

If, while migrating one or more virtual machines, Workload Balancing determines that increasing capacity would benefit the pool’s overall performance, it powers on hosts automatically or recommends doing so.


Workload Balancing never recommends powering on a host unless Workload Balancing powered it off.

Designing Environments for Power Management and VM Consolidation

When you are planning a XenServer implementation and you intend to configure automatic VM consolidation and power management, consider your workload design. For example, you may want to:

  • Place Different Types of Workloads in Separate Pools. If you have an environment with distinct types of workloads (for example, user applications versus domain controllers) or types of applications that perform better with certain types of hardware, consider if you need to locate the virtual machines hosting these workloads in different pools.

    Because power management and VM consolidation are managed at the pool level, you should design pools so they contain workloads that you want consolidated at the same rate, factoring in considerations such as those discussed in the Advanced Settings topic.

  • Exclude Hosts from Workload Balancing. Some hosts may need to be on at all times. See Excluding Hosts from Recommendations for more information.

To apply optimization recommendations automatically

  1. In the Resources pane of XenCenter, select XenCenter > your-resource-pool.
  2. In the Properties pane, click the WLB tab.
  3. In the WLB tab, click Configure WLB.
  4. In the left pane, click Automation.
  5. Select one or more of the following check boxes:
    • Automatically apply Optimization recommendations. When you select this option, you do not need to accept optimization recommendations manually. Workload Balancing automatically accepts optimization and placement recommendations it makes.
    • Automatically apply Power Management recommendations. The behavior of this option varies according to the pool’s optimization mode:
      • Maximum Performance Mode. When Automatically apply Power Management recommendations is enabled, Workload Balancing automatically powers on hosts when doing so improves host performance.
      • Maximum Density Mode. When Automatically apply Power Management recommendations is enabled, Workload Balancing automatically powers off hosts when resource utilization drops below the Low threshold (that is, Workload Balancing powers hosts off automatically during low usage periods).
  6. Do one of the following:
    • If you want to configure power management, click Automation/Power Management and proceed to the section below.
    • If you do not want to configure power management and you are finished configuring automation, click OK.

To select servers for power management

  1. In the Power Management section, select the hosts that you want Workload Balancing to power on and off automatically.


    Selecting hosts for power management recommendations without selecting the Automatically apply Power Management recommendations check box results in Workload Balancing suggesting power management recommendations but not applying them automatically for you.

  2. Click OK. If none of the physical servers in the resource pool support remote power management, Workload Balancing displays the message, No hosts support Power Management

Optimizing and Managing Power Automatically