ESXi host maintenance mode fails due to insufficient memory for VM reservations

Products

• VMware vSphere ESXi

Issue/Introduction

ESXi hosts fail to enter maintenance mode during upgrade operations. vCenter tasks and events indicate insufficient available RAM on the cluster to accommodate VM migrations. The cluster fails to evacuate the host because concurrent maintenance mode activities restrict the pool of available destination hosts.

Environment

7.0.x

Cause

Destination hosts lack the necessary unreserved memory to satisfy migrating VM memory reservations. VMs configured with high memory reservations consume the majority of host capacity. When multiple hosts enter maintenance mode simultaneously during an upgrade, the cluster's available unreserved memory drops below the threshold required to accept inbound vMotion migrations for these reserved VMs.

Resolution

1. Retry the host maintenance mode task or upgrade activity to allow vSphere DRS to attempt a different evacuation sequence.
   
   
2. If the evacuation failure persists, temporarily disable/reduce the memory reservations on the affected VMs to allow vSphere DRS to migrate workloads using active memory metrics.
   
   
3. If strict memory reservations must be maintained and cannot be reduced/disabled, increase total cluster compute capacity by adding new ESXi hosts prior to initiating cluster maintenance operations.