Fault domains (FD) provide the core functionality of vSAN stretched cluster. The maximum number of fault domains in a vSAN stretched cluster is 3. The first Fault Domain can be referred as Preferred data site, the second Fault Domain can be referred as Secondary data site, and the third Fault Domain is the witness host site. It is important to keep utilization per data site below 50% to ensure proper availability, if either the Preferred or Secondary site goes offline.
