I’ve seen some people reporting a strange message in the Virtual SAN UI. The UI states the following: Disk format version 4.0 (update to 2.0 suggested). This is what that looks like (stole the pic from VMTN, thanks Phillip.)
A bit strange considering you apparently have 4.0 why would you go to 2.0 then? Well you are actually on 2.0 and are supposed to go to 3.0. The reason this happens is because, most likely, not all hosts within you cluster are on the same version of Virtual SAN, or vCenter Server was not updated to the last version and ESXi has a higher version. So far I have seen this being reported when people upgrade to vSphere 6.0 Update 2. If you are upgrading, make sure to upgrade all hosts to ESXi 6.0 Update 2, but before you do, upgrade the vCenter Server to 6.0 Update 2 first!