High memory utilization alarms observed on ESXi hosts and cluster despite low active memory usage.

Products

• VMware vCenter Server
• VMware vSphere ESXi

Issue/Introduction

This article explains why high memory utilization alarms are triggered on ESXi hosts or clusters even when the active memory usage is low, and provides steps to address or prevent such alarms.

• vCenter displays “Host memory usage” alarms on one or more ESXi hosts in the cluster.
• The alarms reappears even after being reset or acknowledged.
• From the vCenter UI > ESXi Summary tab, the host shows:
  • High memory usage percentage (above 85%).
  • Active memory usage is only around 3–4% for most virtual machines.
• Example scenario:
  • Total host memory capacity: 766 GB
  • Total VM configured memory: ~756 GB across 9 VMs
  • Average active memory utilization: 3–4%

Environment

• VMware vCenter Server Appliance 8.x
• VMware vSphere ESXi 8.x

Cause

This is an expected behavior based on how vCenter calculates and triggers the “Host Memory Usage” alarm.

• The Host Memory Usage alarm is triggered based on total memory consumption, which includes:
  • Memory allocated to all virtual machines.
  • VMkernel and hypervisor overhead memory
• The default threshold for the “Host Memory Usage” alarm is:
  • Warning: 85%
  • Critical: 95%
• Since the host memory is almost completely allocated to VMs (756 GB allocated out of 766 GB total), the memory usage value exceeds the warning threshold, triggering the alarm.
• Active memory refers to the amount of memory actively used by the guest OS and applications and does not directly affect the alarm trigger.

Resolution

This is an expected design behavior. You can take one of the following actions to mitigate unnecessary alarm triggers:

1. Modify the alarm threshold in vCenter:
   • Go to vCenter Server -> Configure -> Alarm Definitions.
   • Locate “Host Memory Usage”.
   • Edit the threshold values to higher levels (for example, set Warning = 90% and Critical = 97%).
2. Optimize VM memory allocation:
   • Review and adjust VM memory configurations to match the actual workload requirements.
   • Avoid over-provisioning memory to virtual machines that do not require large allocations.
3. Monitor active memory metrics:
   • Use vCenter Performance Charts or ESXTOP to monitor the Active Memory parameter for individual VMs.
   • Focus on active memory trends rather than total allocated memory for accurate performance insights.

Note : This does not indicate a memory leak or performance degradation unless active memory consumption is consistently high.

Additional Information

For additional details on ESXi memory management and performance troubleshooting, consult the following resources.

• ESXi memory analysis using batch ESXTOP for high memory consumption troubleshooting
• Understanding esxtop Memory parameters
• Virtual machine memory limits and hardware versions
• Preconfigured vSphere Alarms

Related memory troubleshooting scenarios :

• Troubleshooting ESX/ESXi virtual machine performance issues
• ESXi Host Memory Swapping
• Performance Best Practices for VMware vSphere 8.0

Vsan can be a significant contributor to vmkernel overhead memory consumption and reservation. The following vSAN resizer tool can be used to get an estimate of the amount of memory vSAN will reserve for its operation:

• https://vcf.broadcom.com/tools/vsansizer/