A LUN Is displaying 90% used space in the NetApp ONTAP CLI, but a Windows Server shows only 10% used space. What is the first step to take to address…

The described scenario, where the ONTAP system reports high LUN space usage while the host operating system reports low usage, is a classic symptom of failed space reclamation. When data is deleted within the Windows file system, the host must inform the storage array that these blocks are now free. This is accomplished using SCSI UNMAP commands. For ONTAP to process these commands, the "space allocation" feature must be enabled on the LUN. Enabling this feature is the necessary first step on the storage system to allow it to reclaim the freed space and align its space usage reporting with the host's view.

A. Enable automatic resizing.

Automatic resizing adjusts the LUN or volume size based on usage thresholds; it does not reclaim space from deleted blocks within the LUN itself.

B. Disable automatic resizing.

This is unrelated to the core issue of space reclamation and would have no effect on the discrepancy in reported space usage.

C. Disable space allocation.

This would explicitly prevent ONTAP from processing SCSI UNMAP commands, making it impossible to reclaim space and resolve the issue.

1. NetApp ONTAP Documentation

lun modify command: The -space-allocation parameter is described as controlling whether space allocation for the LUN is enabled. The description states

"If this parameter is set to enabled

the blocks are allocated and unallocated on demand by the host." This directly links the setting to the host's ability to unallocate (reclaim) blocks.

Source: NetApp ONTAP 9 Commands

lun modify man page.

2. NetApp ONTAP SAN Administration Guide

"Enable space allocation and reclamation for a LUN": "You can enable space allocation on a LUN to enable the host to reclaim space. When space allocation is enabled

the storage system can receive SCSI UNMAP commands from the host to deallocate blocks that are no longer in use."

Source: NetApp ONTAP 9.10.1 Documentation

SAN Administration Guide

Section: "Enable space allocation and reclamation for a LUN".

3. NetApp Technical Report TR-4336

"Best Practices for Scalable SAN on NetApp ONTAP": Section 3.2

"Space Reclamation

" explains: "For space reclamation to work

it must be supported and enabled on the host OS

the host file system

and the storage array... On the ONTAP LUN

the space-allocation option must be enabled."

Source: NetApp TR-4336

Page 11

Section 3.2 "Space Reclamation".