Changing the Critical Thresholds

This topic provides guidance about how to modify the default Critical thresholds and how to set values for the Critical threshold alter High, Medium, and Low thresholds.

This information is only provided for reference while changing thresholds. To understand the concepts discussed in this topic, it is important to read them in the fuller context of the information provided in the Workload Balancing documentation.


When evaluating utilization, Workload Balancing compares its daily average to four thresholds: low, medium, high, and critical. After you specify (or accept the default) critical threshold, Workload Balancing sets the other thresholds relative to the critical threshold on a pool. You might want to change Critical thresholds as a way of controlling when optimization recommendations are triggered.

Workload Balancing evaluates CPU, Memory, Network Read, Network Write, Disk Read, and Disk Write utilization for physical hosts in a resource pool.

Workload Balancing determines whether to recommend relocating a workload and whether a physical host is suitable for a virtual-machine workload by evaluating:

  • Whether a resource’s critical threshold is met on the physical host
  • (If the critical threshold is met) the importance assigned to a resource


To prevent data from appearing artificially high, Workload Balancing evaluates the daily averages for a resource and smooths utilization spikes.

Workload Balancing determines whether to produce recommendations based on if the averaged historical utilization for a resource violates its threshold. Workload Balancing recommendations are triggered when the High threshold in Maximum Performance mode or Low and Critical thresholds for Maximum Density mode are violated.

After you specify a new Critical threshold for a resource, Workload Balancing resets the resource’s other thresholds relative to the new Critical threshold. (To simplify the user interface, the Critical threshold is the only threshold you can change through XenCenter.)

For more information, see Workload Balancing documentation.

Default settings for thresholds

The following table shows the default values for the Workload Balancing thresholds:

Setting Default High Medium Low
CPU Utilization 90% 76.5% 45% 22.5%
Free Memory 51 MB 63.75 MB 510 GB 1,020 GB
Network Read 25 MB/s 21.25 MB/s 12.5 MB/s 6.25 MB/s
Network Write 25 MB/s 21.25 MB/s 12.5 MB/s 6.25 MB/s
Disk Read 25 MB/s 21.25 MB/s 12.5 MB/s 6.25 MB/s
Disk Write 26 MB/s 21.25 MB/s 12.5 MB/s 6.25 MB/s

To calculate the values for the High, Medium, and Low resource metrics, Workload Balancing multiplies the new value for the Critical threshold with the following factors:

  • High Threshold Factor: 0.85
  • Medium Threshold Factor: 0.50
  • Low Threshold Factor: 0.25

To calculate threshold values for free memory, Workload Balancing multiplies the Critical threshold with these factors:

  • High Threshold Factor: 1.25
  • Medium Threshold Factor: 10.0
  • Low Threshold Factor: 20.0

This behavior means that if you increase, for example, a Critical threshold to 95%, WLB automatically resets the other thresholds to the following:

  • High threshold to 80.75%
  • Medium threshold to 47.5%
  • Low threshold to 23.75%

To perform this calculation for a specific threshold, multiply the factor for the threshold with the value you entered for the critical threshold for that resource:

High, Medium, or Low Threshold = Critical Threshold \* Threshold Factor

For example, if you change the Critical threshold for Network Reads to 40 MB/s and you want to know its other thresholds:

  • To obtain the Low threshold, multiply 40 by 0.25
  • To obtain the Medium threshold, multiply 40 by 0.50
  • To obtain the High threshold, multiply 40 by 0.85

To prevent the pool coordinator from becoming overloaded, Workload Balancing automatically sets the pool coordinator’s Critical Thresholds at lower values.

How other thresholds trigger recommendations

While the Critical threshold triggers many recommendations, other thresholds can also trigger recommendations, as follows:

High threshold

  • Maximum Performance. Exceeding the High threshold triggers optimization recommendations to relocate a virtual machine to a host with a lower resource utilization.
  • Maximum Density. Workload Balancing doesn’t recommend placing a VM on host if doing so causes the utilization of any host resource to exceed the High threshold value.

Low threshold

  • Maximum Performance. Workload Balancing does not trigger recommendations from the Low threshold.
  • Maximum Density. When a metric value drops below the Low threshold, it signals Workload Balancing that hosts are being underutilized. This signal triggers an optimization recommendation to consolidate virtual machines on fewer hosts. Workload Balancing continues to recommend moving virtual machines onto a host until the metric values for one of the host’s resources reaches its High threshold.

    However, if after a VM is relocated, utilization of a resource on the new host exceeds its Critical threshold, WLB temporarily uses an algorithm similar to the Maximum Performance algorithm to find a new host for the VMs. Workload Balancing continues to use this algorithm to recommend moving virtual machines until resource utilization on hosts across the pool falls below the High threshold.

To change the critical thresholds

  1. Select the pool in the Resources pane, select the WLB tab, and then select Settings.
  2. In the left pane, select Critical Thresholds.
  3. In the Critical Thresholds page, accept or enter a new value in the Critical Thresholds boxes. Workload Balancing uses these thresholds when making virtual-machine placement and pool-optimization recommendations. Workload Balancing strives to keep resource utilization on a host below the critical values set.
Changing the Critical Thresholds