Stretched clusters extend the vSANcluster from a single data site to two sites for a higher level of availability and inter site load balancing. Stretched clusters are typically deployed in environments where the distance between data centers is limited, such as metropolitan or campus environments.
You can use stretched clusters to manage planned maintenance and avoid disaster scenarios, because maintenance or loss of one site does not affect the overall operation of the cluster. In a stretched cluster configuration, both data sites are active sites. If either site fails, vSANuses the storage on the other site. vSphere HA restarts any VM that must be restarted on the remaining active site.
You must designate one site as the preferred site. The other site becomes a secondary or nonpreferred site. The system uses the preferred site only in cases where there is a loss of network connection between the two active sites. The site designated as preferred typically is the one that remains in operation, unless the preferred site is resyncing or has another issue. The site that leads to maximum data availability is the one that remains in operation.
Each stretched cluster consists of two data sites and one witness host. The witness host resides at a third site and contains the witness components of virtual machine objects. It contains only metadata, and does not participate in storage operations.
The witness host serves as a tiebreaker when a decision must be made regarding availability of datastore components when the network connection between the two sites is lost. In this case, the witness host typically forms a vSANcluster with the preferred site. But if the preferred site becomes isolated from the secondary site and the witness, the witness host forms a cluster using the secondary site. When the preferred site is online again, data is resynchronized to ensure that both sites have the latest copies of all data.
More information on VMware vSAN Stretched Cluster can be found here.