Disparity between the disk space reported by vCenter/ESXi and the actual used space reported by the NetApp array

Products

• VMware vSphere ESXi

Issue/Introduction

The vCenter and ESXi are reporting the amount of Disk Space Used/Consumed as higher than that reported by the NetApp storage array. 

Environment

Esxi 7.x and above.

VCenter 7.x and above

Cause

The disparity between the disk space reported by vCenter/ESXi and the actual used space reported by the NetApp array is normal and expected behavior due to differences in how each system measures storage, primarily concerning thin provisioning and space reclamation. 

Resolution

vCenter/ESXi Perspective (Logical View): 

• VMware generally reports space from a logical perspective.

• When you create a thin-provisioned VMDK, vCenter deducts the full provisioned size from the available space in its metadata, even if the VM has only written a small amount of data.

• For example, if you create a 300 GB thin-provisioned disk, vCenter views 300 GB as "potentially used" space that is no longer freely available to other VMs on that datastore, to prevent over-allocation issues from the VMware side.

• Once a VM writes data to a block, that block is considered "in use" by the hypervisor and remains so, even if the data is deleted within the guest OS.

• Other files, such as VM swap files (which are created when a VM is powered on and are sized according to the VM's assigned memory), also contribute to the space vCenter marks as used.

 

NetApp Perspective (Physical View): 

• The NetApp storage array, using its own thin provisioning capabilities (often deployed in FlexVol volumes), reports the actual physical disk space consumed by data written to the LUN.

• The array is only concerned with the blocks that have physically received data (actual 1s and 0s). If a 300 GB VMDK only has 80 GB of actual data written to it, the NetApp array will correctly show only about 80 GB as used physical space.

• Space-saving technologies like deduplication and compression on the NetApp side can further reduce the physical space used, making the physical usage even lower than what ESXi sees. 

Additional Information

The NetApp is reporting the physical consumption, while vCenter is reporting the logical allocation (or the maximum potential consumption). The size disparity would only be an issue if the Array was showing more used than the esxi hosts. This could indicate a reclamation issue.