Trying to upgrade a 2 node cluster shows in vCenter: "insufficient memory resources on standby hosts (if any) to meet the requirements of a given operation"

Products

• VMware vSphere ESXi

Issue/Introduction

Trying to upgrade ESXi 7.0U3 to 8.0U3 leads to error: "host #### reported an issue which prevents entering maintenance mode: insufficient memory resources on standby hosts (if any) to meet the requirements of a given operation"

Environment

ESXi 7.x, 8.x

Cause

Admission Control Policies don't allow host to enter maintenance mode

vSphere HA’s "Specify a failover host" or "Cluster Resource Percentage" policies reserve resources for failover scenarios. In small clusters (especially 2-node setups), placing one host in maintenance mode leaves insufficient reserved resources for HA’s requirements.

Example: A 2-node cluster with HA enabled cannot tolerate losing the second host if one is in maintenance mode, triggering the error

Resolution

Disable HA Admission Control Temporarily:
For 2-node clusters, disable HA before remediation to bypass resource reservation checks.

Steps:

1. Right-click the cluster in vCenter Server and choose Settings.
2. In Configure tab, Select "vSphere Availability" Click EDIT for vSphere HA.
3. Select Admission Control.
4. Select "Disabled" for define host failover capacity by

Re-enable HA after the upgrade.