After Workload Balancing is running for a period of time, if you do not receive optimal placement recommendations, Citrix strongly recommends you evaluate your performance thresholds as described in the Workload Balancing Administrator's Guide. It is critical to set Workload Balancing to the correct thresholds for your environment or its recommendations might not be appropriate.
You can specify the number of minutes Workload Balancing must wait after the last time a particular virtual machine was moved, regardless of the cause, before it can generate another optimization recommendation that includes that particular virtual machine.
The recommendation interval is designed to prevent Workload Balancing from generating recommendations for artificial reasons (for example, if there was a temporary utilization spike).
Every two minutes, Workload Balancing checks to see if it can generate recommendations for the pool it is monitoring. When you enable Automation, you can specify the number of times a consistent recommendation must be made before Workload Balancing can automatically apply the recommendation. To do so, you configure a setting known as the Recommendation Count. The Recommendation Count and the Optimization Aggressiveness setting let you fine-tune the automated application of recommendations in your environment.
As described in the overview section, Workload Balancing uses the similarity of recommendations to a) vet if the recommendation is truly needed and b) determine if the destination host has stable enough performance over a prolonged period of time to accept a relocated virtual machine (without needing to move it off the host again shortly). Workload Balancing uses the Recommendation Count value to determine a recommendation must be repeated before Workload Balancing automatically applies the recommendation.
Accepting recommendations uses system resources and affects performance when Workload Balancing is relocating the virtual machines. Increasing the Recommendation Count increases the number of matching recommendations that must occur before Workload Balancing applies the recommendation, which encourages Workload Balancing to apply more conservative, stable recommendations and may decrease the potential for spurious virtual machine moves. It should be noted, however, that the Recommendation Count is set to a conservative value by default.
Because of the potential impact adjusting this setting may have on your environment, Citrix only recommends changing it with extreme caution, preferably by testing and iteratively changing the value or under the guidance of Citrix Technical Support.
All optimization recommendations include a severity rating (Critical, High, Medium, Low) that indicates the importance of the recommendation. Workload Balancing bases this rating on a combination of factors including configuration options you set, such as thresholds and metric tunings; resources available for the workload; and resource-usage history.
When you configure Workload Balancing to apply optimization recommendations automatically, you can set the minimum severity level that should be associated with a recommendation before Workload Balancing automatically applies it.
To provide additional assurance when running in automated mode, Workload Balancing has consistency criteria for accepting optimizations automatically so as to prevent moving virtual machines due to spikes and anomalies. In automated mode, Workload Balancing does not accept the first recommendation it produces. Instead, Workload Balancing waits to automatically apply a recommendation until a host or virtual machine exhibits consistent behavior over time. The phrase consistent behavior over time refers to factors such as whether a host continues to trigger recommendations and whether the same virtual machines on that host continue to trigger recommendations.
Workload Balancing determines if behavior is consistent by using criteria for consistency and by having criteria for the number of times the same recommendation is made (that is, the Recommendation Count). You can configure how strictly you want Workload Balancing to apply the consistency criteria using a Optimization Aggressiveness setting.
While Citrix primarily designed the Optimization Aggressiveness setting for demonstration purposes, you can use this setting to control the amount of stability you want in your environment before Workload Balancing applies an optimization recommendation. The most stable setting (Low aggressiveness) is configured by default. In this context, the term stable refers to the similarity of the recommended changes over time, as explained throughout this section.
Workload Balancing uses up to four criteria to ascertain consistency. The number of criteria that must be met varies according to the level you set in the Optimization Aggressiveness setting. The lower the level (for example, Low or Medium) the less aggressively Workload Balancing is in accepting a recommendation. In other words, Workload Balancing is stricter about requiring criteria to match (or less cavalier or aggressive) about consistency when aggressiveness is set to Low.
For example, if the aggressiveness level is set to Low, Workload Balancing requires that each criterion for Low is met the number of times specified in the Recommendations box (where you specify the Recommendation Count value) before automatically applying the recommendation.
For example, if you set the Recommendation Count in the Recommendations box to "3", you are making Workload Balancing wait until it sees all the criteria listed in the Workload Balancing Administrator's Guide for Low are met and repeated in three consecutive recommendations. This helps ensure that the virtual machine actually needs to be moved and that the destination host Workload Balancing is recommending has consistently stable resource utilization over a longer period of time. It reduces the potential for a recently moved virtual machine to be moved off a host due to host performance changes after the move. By default, this setting is set to a conservative setting (Low) to encourage stability.
Citrix does not recommend increasing the Optimization Aggressiveness to increase the frequency with which your hosts are being optimized. If you feel that your hosts are not being optimized quickly or frequently enough, try adjusting the Critical thresholds, as described in Changing the Critical Thresholds.
For details about the consistency criteria associated with the different levels of aggressiveness, see the Workload Balancing Administrator's Guide.
If you find that Workload Balancing is not automatically applying optimization recommendations frequently enough, you might want to increase the aggressiveness setting. However, Citrix strongly recommends reviewing the information in the Workload Balancing Administrator's Guide before doing so.
- In the Minutes box, type a value for the number of minutes you want Workload Balancing to wait before making another optimization recommendation on a newly rebalanced server.
- In the Recommendations box, type a value for the number of optimization recommendations you want Workload Balancing to make before it applies an optimization recommendation automatically.
- Select a minimum severity level before optimizations are applied automatically.
- Modify how aggressively Workload Balancing automatically applies optimization recommendations.