Addressing vCPU Allocation Imbalance in vSAN Clusters

Products

VMware vSAN

Issue/Introduction

Symptoms:

In a vSAN OSA cluster running ESXi 8.0 U3, administrators may observe a significant disparity in vCPU allocation across hosts despite Distributed Resource Scheduler (DRS) being enabled in "Fully Automated" mode.
While some hosts remain nearly idle, others may show vCPU-to-pCPU ratios exceeding 100%, causing a perceived workload imbalance.

Issue Verification

In the affected environment, a 4-host cluster shows the following distribution:

Host Name	vCPUs Used	Available pCPUs	Allocation Ratio	Status
vsan-prod-01	587	336	175%	Yellow
vsan-prod-03	439	336	131%	Yellow
vsan-prod-02	4	336	1%	Green
vsan-prod-04	6	336	2%	Green

Environment Profile:

vMotion: Enabled on all hosts.
vSphere HA: Enabled.
DRS: Fully Automated (Migration Threshold 3).
Affinity Rules: None present.

Environment

VMware vSAN 8.x

Cause

This behavior is by design.

Standard DRS logic prioritizes active resource demand (actual CPU cycles consumed) over static allocation (configured vCPUs). If the VMs on the highly allocated hosts (01 and 03) are currently idle or have low utilization, DRS detects no resource contention.
At Migration Threshold 3, the "cost" of a vMotion (network and compute overhead) is calculated to be higher than the projected performance benefit of moving an idle VM. Consequently, DRS will not trigger migrations simply to "balance the colors" of allocation ratios.

Resolution

Implementing CPU Over-Commitment

To force DRS to balance the cluster based on vCPU allocation rather than active demand, you must configure the CPU Over-Commitment constraint. This creates a mandatory rule that DRS must follow, regardless of active utilization.

Steps to Configure:

Navigate to the vSphere Cluster in the vSphere Client.
Go to the Configure tab.
Under Services, select vSphere DRS and click Edit.
Expand Additional Options.
Enable CPU Over-commitment.
Set the vCPU:pCPU ratio threshold (e.g., 1.5:1 or 2:1).
- Note: Setting a stricter ratio prevents any single host from vastly exceeding the allocation levels of its peers.
Click OK.

By setting a cluster-level CPU Over-Commitment ratio, you transform allocation balance from a "recommendation" into a mandatory constraint.

Action: DRS will proactively migrate VMs from the over-allocated hosts (01 and 03) to the under-utilized hosts (02 and 04) to satisfy the new ratio.
Result: The cluster will achieve a "Normal" state with a significantly improved DRS Score and an even distribution of vCPU-to-pCPU ratios across all four hosts.
Important: Ensure your vMotion network has sufficient bandwidth to handle the initial wave of migrations required to rebalance the cluster after applying these settings.