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"I have removed the offending 6.7 host from the cluster, and removed the 6.5 host as well. I change the Virsto.DiskFormatVersion on the new hosts to 5, and re-added the 6.5 host, but it can still not see anything on the vsanDatastore."
How exactly was the 6.5 node removed and re-added to the cluster? e.g. does it still have its original Disk-Groups intact and healthy?
If you didn't recreate the Disk-Groups on re-joining the cluster check using:
#esxcli vsan storage list | grep CMMDS
Are you positive you didn't perform On-Disk format for the remaining 6.7 nodes in the absence of the 6.5 node?
Can check this via GUI but more clear from RVC:
> vsan.disks_stats <pathToCluster>
> vsan.obj_status_report <pathToCluster>
Sounds like the 6.5 node is potentially isolated from the cluster - if the 'Remediate cluster' button is clickable via the 'vCenter is authoritative' Health check under Cluster then potentially you can remediate unicastagent entries automatically - otherwise make the necessary checks e.g. node has vSAN-enabled vmk(s) that can communicate with the other nodes vSAN-enabled vmk(s), all nodes in cluster have 6.5 node entry in their unicastagent lists, 6.5 node has unicastagent list entries for all other nodes, node is out of Maintenance Mode, node is not having some other issues such as controller physical/driver/firmware issue (#dmesg is your friend here).
I am just rebuilding, I appreciate the response though. I already began.
I am well aware I am running in a very unsupported config, and all of my VMs became inaccessible.