With a vSphere 5 upgrade there is an important vDesign decision: if you already have some VMFS3 datastores could be better upgrade them to the new version of build new datastores directly with VMFS5? The upgrade procedure is quite fast and friendly and could be applied to a live datastore, so seems that there isn’t a big different between an upgrade or a clean format. But usually the recommendation is to re-format each LUN to VMFS-5 rather than upgrade it. This will fix a number of issues, including: