Q: How are VMware vSphere's DRS Migration Threshold and <em>Target host load standard deviation</em> settings related?

A: vSphere Distributed Resource Scheduler (DRS) clusters have multiple configuration options that impact cluster behavior. One of those settings is the cluster's Automation Level, found under VMware DRS when viewing cluster properties. Three settings are available for Automation Level: Manual, Partially Automated, and Fully Automated.

When you select Fully Automated, a slider appears for setting the cluster's Migration Threshold. That threshold can be set between Conservative on the left and Aggressive on the right. Setting the slider to Conservative instructs the cluster to apply only priority 1 migration recommendations, and setting it to Aggressive instructs it to apply priority 1 through priority 5 recommendations. Selections between these two values apply recommendations scaling up from fewer on the Conservative end to more on the Aggressive end.

The Migration Threshold selection is directly related to the value for Target host load standard deviation, which is found under the cluster's Summary tab in the VMware DRS box. A more conservative Migration Threshold will drive a higher value for Target host load standard deviation, which will in turn define fewer VM migrations that an unbalanced cluster will apply to return that cluster to balance.

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