OK. So vSAN requires a minimum of 3 fault domains. IF you do not manually configure Fault Domains (FDs), then each host is implicitly a Fault Domain. So if you have 4 hosts in a vSAN cluster, then each host is its own FD. vSAN can sustain a failure of a fault domain when speaking about hosts (within a host a drive failure is 1 failure, etc.) with FTT1.
If you are have 4,6,12, or 16 Fault Domains you can only sustain 1 failure, IF you have the default policy of FTT=1 regardless of FTM (RAID1 or RAID5).
If you want to be able to sustain 2 failures, and have 6 host or 6 FDs, then you can do FTT=2, and still have full redundancy. Otherwise, if you have 6 FDs with FTT=1, and have 2 failures, some of the data will be unavailable.
Fault domains are used so that vSAN knows where to spread the data while maintaining compliance of the policy for failures to tolerate. Also remember that the policies are applied to the object level, so you can have an object with FTT=2, and another object with FTT=1.
"They can define the first fault domain for the first 4 nodes and when they add the second set of 4 nodes they can define the second fault domain with those 4 nodes and so on"
-- No. You cannot have only one FD. If you have 4 hosts, you will have 4 implicit FDs, or you will create 4 FDs.
Makes sense?
