Product Documentation

Changing the Critical Thresholds

May 03, 2013
Important: Workload Balancing (WLB) and its functionality has been removed from XenServer version 6.2 and higher. Applications, code, or usage that depend WLB will not function in XenServer version 6.2 and higher. See CTX137333 for details.

This topic provides guidance about how to modify the default Critical thresholds and how values set for 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 Administrator's Guide.


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

For more information, see the Workload Balancing Administrator's Guide.

Workload Balancing determines whether or not to produce recommendations based on if the averaged historical utilization for a resource violates its threshold. As discussed in the Workload Balancing Administrator's Guide, 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.)

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 51MB 63.75MB 510GB 1020GB
Network Read 25MB/sec 21.25MB/sec 12.5MB/sec 6.25MB/sec
Network Write 25MB/sec 21.25MB/sec 12.5MB/sec 6.25MB/sec
Disk read 25MB/sec 21.25MB/sec 12.5MB/sec 6.25MB/sec
Disk write 26MB/sec 21.25MB/sec 12.5MB/sec 6.25MB/sec
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 means that if you increase, for example, the pool's Critical threshold for CPU Utilization to 95%, Workload Balancing automatically resets the High, Medium, and Low thresholds to 80.75%, 47.5%, and 23.75% respectively.

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 40MB/sec and you want to know its Low threshold, you multiply 40 by 0.25, which equals 10MB/sec. To obtain the Medium threshold, you multiple 40 by 0.50, and so on.

To prevent the pool master from becoming overloaded, Workload Balancing automatically sets the pool master'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 lower resource utilization.
  • Maximum Density. Workload Balancing will not recommend placing a virtual machine on host if moving that virtual machine to the host will cause the utilization of any of the host's resources 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 and 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 resource reaches its High threshold.

    However, if after a virtual machine is relocated, a resource's utilization on the virtual machine's new host exceeds its Critical threshold, Workload Balancing will temporarily use an algorithm similar to the Maximum Performance load-balancing algorithm to find a new host for the virtual machines. 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, click on the WLB tab, and then click Settings.
  2. In the left pane, select Critical Thresholds.
  3. In 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.