DRS migrates VMs when the cluster appears/shows balanced after upgrading to vCenter Server 6.0 U3 or 6.5

Products

• VMware vCenter Server

Issue/Introduction

• DRS performs too many migrations after upgrading to vCenter Server 6.0 U3 or later versions.
• DRS cluster appears/shows balanced.

Environment

• VMware vCenter Server Appliance 6.0.x
• VMware vCenter Server 6.0.x
• VMware vCenter Server Appliance 6.5.x
• VMware vCenter Server 6.5.x

Cause

Starting from vSphere 6.0 U3 and 6.5, if it detects a pairwise imbalance between two hosts, DRS will continue load balancing after the standard deviation based load balancing is complete. A pairwise imbalance exists when the resource usage of any two hosts differ by more than a threshold (default = 20%). This could result in VM migrations even when the cluster appears balanced according to standard deviation.

Resolution

This issue is resolved in vCenter Server 6.0 U3j and 6.5 U2d. 

Workaround:

Switch back to the old behavior by turning off the Pair-wise balancing by setting the cluster advanced option CheckPairwiseImbalance to 0.

Note: Advanced option "CheckPairwiseImbalance" is case sensitive.

This will disable this enhancement and revert to the same behavior as before.