Workload Balancing uses metric weightings, a method of assigning importance to resources, to determine what hosts it should optimize first.
The way in which Workload Balancing uses metric weightings when determining which hosts and VMs to process first varies according to the optimization mode, Maximum Density or Maximum Performance.
When Workload Balancing is processing optimization recommendations, it creates an optimization order. To determine this, Workload Balancing ranks the hosts to address first according to which hosts have the highest metric values for whatever resource is ranked as the most important in the metric weightings page.
In general, metric weightings are mainly used when a pool is in Maximum Performance mode. However, when Workload Balancing is in Maximum Density mode, it does use metric weightings if a resource exceeds its Critical threshold.
In Maximum Performance mode, Workload Balancing uses metric weightings to determine (a) which hosts' performance to address first and (b) which VMs to recommend migrating first.
For example, if you rank Network Writes as the most important resource for Workload Balancing to consider—that is, you move the Metric Weighting slider all the way to the right (More Important) and you move all the other sliders to the middle—then Workload Balancing will begin addressing performance issues (that is, making optimization recommendations) on the host with the highest number of Network Writes per second first.
In Maximum Density mode, Workload Balancing only uses metric weightings when a host reaches the Critical threshold—then Workload Balancing applies a Maximum Performance-like algorithm until no Hosts are exceeding the Critical thresholds. When using the Maximum Performance-like algorithm, Workload Balancing uses metric weightings to determine the optimization order in the same way as it does for Maximum Performance mode.
If two or more hosts have resource(s) exceeding their Critical thresholds, Workload Balancing verifies the importance you set for each resource before determining which host to optimize first and which VMs on that host to relocate first.
Workload Balancing will recommend optimizing Host A first because the resource on it that reached the Critical threshold is the resource assigned the highest weight. After Workload Balancing determines that it needs to address the performance on Host A, Workload Balancing then begins recommending placements for VMs on that host beginning with the VM that has the highest CPU utilization, since that CPU utilization is the resource with the highest weight.
After Workload Balancing has recommended optimizing Host A, it makes optimization recommendations for Host B. When it recommends placements for the VMs on Host B, it does so by addressing CPU utilization first, since CPU utilization was assigned the highest weight.
If there are additional hosts that need optimization, Workload Balancing addresses the performance on those hosts according to what host has the third highest CPU utilization.
Moving the slider towards Less Important indicates that ensuring virtual machines always have the highest amount of this resource available is not as vital on this resource pool.