Mismatched VMDK and Guest OS Disk Usage on vSAN Datastores

Products

• VMware vSAN

Issue/Introduction

A mismatch in disk usage is observed between the virtual machine’s thin-provisioned VMDK and the Guest OS-reported disk usage.

Example:

• Virtual Machine: Virtual_Machine1

• VMDK size: 24 TB (thin-provisioned)

• Datastore usage: 13.56 TB

Guest OS usage: lower than reported datastore usage

Verification from the ESXi host confirms that Guest UNMAP is disabled:

[root@ESXi1:/vmfs/volumes/vsan:52b483f#######-#######24563/e51bb568-####-####-####-9cd57d43d2le] vsish -e get /config/VSAN/intOpts/GuestUnmap
Vmkernel Config Option {
Default value:0
Min value:0
Max value:1
Current value:0
hidden config option:1
Description: Whether Guest Unmap feature is turned on
Host specific config option:0
Option update requires reboot: 0
Option update requires maintenance mode: 0

Environment

VMware vSAN 7.x

VMware vSAN 8.x

Cause

When UNMAP/Trim is disabled, the ESXi host and vSAN datastore cannot reclaim unused storage blocks that are freed inside the Guest OS.
This results in inflated space usage reporting at the datastore level compared to the actual usage within the Guest OS.

Resolution

Enabled UNMAP/Trim in vSAN 

Power cycle the affected VMs to apply the Guest UNMAP setting.

 

Prerequisites for UNMAP support:

• Windows VMs: Minimum virtual hardware version 11

• Linux VMs: Minimum virtual hardware version 13

Once enabled and VMs are restarted, unused blocks released inside the Guest OS will be reclaimed by vSAN, aligning VMDK usage with Guest OS usage.

 

Additional Information

 vSAN Space Efficiency Technologies