ESXi uses different approaches to VMFS5 and VMFS3 upgrades.

VMFS5 Datastores

You cannot upgrade a VMFS5 datastore to VMFS6. If you have a VMFS5 datastore in your environment, create a VMFS6 datastore and migrate virtual machines from the VMFS5 datastore to VMFS6.

VMFS3 Datastores

ESXi no longer supports VMFS3 datastores. The ESXi host automatically upgrades VMFS3 to VMFS5 when mounting existing datastores. The host performs the upgrade operation in the following circumstances:
  • At the first boot after an upgrade to ESXi 6.7 or later, when the host mounts all discovered VMFS3 datastores.
  • When you manually mount the VMFS3 datastores that are discovered after the boot, or mount persistently unmounted datastores.
When you have VMFS3 datastores in your environment, the following considerations apply:
  • If you use an ESXi .iso image to upgrade your legacy host through vSphere Update Manager, and the upgrade is not successful, the VMFS3 datastore is upgraded to VMFS5 if the installation process passes the mount phase.
  • In the mixed environment with the ESXi hosts of the 6.5 release and 6.7 and later, the VMFS3 datastore upgrades when the 6.7 host attempts to mount it. The 6.5 host can continue to access the datastore even when the upgrade is unsuccessful.
  • When you mount the VMFS3 datastore after its resignaturing, it does not upgrade to VMFS5. You must perform the upgrade manually.