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