NFS Datastore Reports Incorrect Free Space Due to NetApp Fractional Reserve on VMware vSphere

Products

• VMware vSphere ESXi

Issue/Introduction

This article addresses capacity over-utilization on NFS datastores where the datastore reports significantly less free space than expected. A common indicator of this issue is the vCenter Datastore Space Utilization chart reporting large amounts of space consumed by "Other Files," despite low actual file usage on the datastore.

Symptoms

• The vSphere UI reports critically low free space (e.g., 1.18 TB free on a 7 TB datastore) while actual VM utilization is significantly lower.
• The vCenter Datastore Space Utilization chart displays a large portion of storage categorized as "Other Files."
• Executing du -sh via the ESXi CLI on the NFS mount reports the correct, lower actual file utilization.
• Storage array and vSphere capacity metrics do not align with file-level usage.

Environment

VMware ESXi 8.x

Cause

The backing NetApp storage array has "Fractional reserve" (also known as LUN overwrite reserve) configured to 100% on the affected volume. This setting reserves space on the storage array for snapshot overwrites, causing the array to artificially reduce the available free space reported to the ESXi hosts via the NFS export. This setting is not applicable or recommended for thin-provisioned NAS environments.

Resolution

1. Verify the volume configuration on the NetApp storage array to confirm the "Fractional reserve" setting is enabled.
2. Engage the storage vendor (NetApp) to modify the "Fractional reserve" (LUN overwrite reserve) setting on the volume.
3. Monitor the datastore capacity in vCenter after the change to verify that the free space reporting returns to expected levels.

For further guidance on modifying storage volume attributes and best practices for thin-provisioned NAS environments, please consult your NetApp vendor documentation regarding Fractional reserve settings.