Disk usage discrepancy between Linux OS and Aria Operations

Products

• VCF Operations/Automation (formerly VMware Aria Suite)

Issue/Introduction

In a Linux OS, the output of "df" command reports a certain usage for a partition, e.g. 1% Used.
In Aria Operations, the metric "Guest File System:/partition|Partition Utilization (%)" of the VM shows a higher usage, e.g. 5%.

Environment

Aria Operations 8.18.x

Cause

This is as expected as the OS doesn't report the reserved blocks, while Aria Operations does.

Resolution

Following is the Aria Operations calculation for partition metrics: 

// Partition Capacity (GB)
capacity_gb = capacity / VMwareConstants.GB  // capacity in bytes ÷ 1073741824

// Partition Utilization (GB)
usage_gb = (capacity - freeSpace) / VMwareConstants.GB

// Partition Utilization (%)
usage_percent = ((capacity - freeSpace) / capacity) * 100
 

Capacity and Free Space data are being fetched from vCenter. 

By the following API request, we can get the data which is being published in Aria Operations:
https://<vCenter_FQDN>/mob/?moid=<vm-MoRef>&doPath=guest.disk
Example output: 

This calculation includes reserved blocks ~5%, which is standard reservation number for Linux based OS.
It is a default ~5% of a filesystem that is automatically reserved for the superuser (root) to prevent fragmentation and ensure critical system daemons can continue to run even if the disk is full.

"dh" command excludes the reserved blocks in its calculation, but Aria Operations takes it into account and that is the source of discrepancy between the outcomes.