Virtual machine memory usage in Aria Operations is higher than that in guest os

Products

• VMware Aria Operations (formerly vRealize Operations) 8.x

Issue/Introduction

• Virtual machine memory usage in Aria Operations is observed to be generally higher than that in guest os.
• The discrepancy may be observed on both Windows and Linux virtual machines.

Environment

Aria Operations 8.18.x

Cause

The discrepancy is mainly due to the specific method of how "Memory| Usage(%)" metric is calculated in Aria Operations.

Memory| Usage(%) is calculated from Guest| Needed Memory(KB) metric in Aria Operations. 

Guest |Needed Memory(KB) is not a raw counter from Windows or Linux. This is a derived counter provided by VMware Tools to estimate the memory needed to run with minimum swapping. It's a more conservative estimate as it includes some of the cache. It has a 5% buffer for spike, based on the general guidance from Microsoft, without which the guest os may swap.

As Guest| Needed Memory(KB) adds a 5% buffer as cache, this is generally higher than the actual used memory in guest os. As a result the Memory| Usage(%) is generally higher than that in guest os.

Resolution

The Memory| Usage(%) in Aria Operations is expected to be different from that in guest os due to different data collection and calculation method. 

If it is intended to pull the memory utilization as observed in guest os, it's suggested to use "Guest|Used Memory(KB)" metric which is more similar to memory usage from guest os view.

Additional Information

• For more information on how Memory| Usage(%) metric is calculated in Aria Operations: VM Memory Usage % Discrepancies when compare to vCenter Performance Overview, Guest OS Task Manager, or Linux top command.
• For more information on how Guest |Needed Memory(KB) metric is calculated: VMware vSphere Metrics.