Migration and converting disk from thin to thick provision fails.

Products

• VMware vSphere ESXi

Issue/Introduction

When attempting to storage vMotion to new datastore, while changing VMDK to thick provision, fails with error: 

• The disk convert allocation type operation failed: 38 (Function not implemented).

 

Environment

ESXi 8.0 or later

Cause

This issue occurs in ESXi 8.0 or higher when the source datastore uses the VMFS5 file system. Version 8.0 introduced a 'Convert Allocation' feature that allows disk provisioning changes (Thin to Thick) to happen simultaneously during a Storage vMotion.

 

Limitation: This specific feature is currently not supported on VMFS5 volumes. If a migration requires an inflation operation (changing a Thin VMDK to Thick) from a VMFS5 source, the process fails. However, migrations that do not change the provisioning type, or those that convert from Thick to Thin, remain fully supported.

Resolution

To resolve this issue, perform the migration and disk inflation as two separate operations:

1: Migrate the VM:

• Perform the Storage vMotion while maintaining the existing provisioning type (e.g., Thin to Thin).
  • Note: Converting from Thick to Thin during migration is supported and does not require these extra steps.

2: Power Off the VM:

• The disk inflation process cannot be performed while the Virtual Machine is running.

3: Inflate the Disk via Datastore Browser:

• Navigate to the ESXi host datastore browser and locate the VMDK file.
• Right-click the file and select Inflate to convert it to Thick provisioned.
  • For detailed steps, refer to the VMware guide on changing provisioning.

Note: This is a known issue affecting ESXi 8.0u3 when the source is VMFS5. The fix is planned to be resolved in ESXi 8.0P09 (8.0 u3j).

 

Additional Information

Changing the thick or thin provisioning of a virtual disk