Once you have the ISO file, here is how to deploy the appliance.
on the vSAN datastore it is meant to monitor. It should reside on a separate physical host or a different vCenter environment if using disaggregated storage. Dell VxRail Architecture Overview | Dell US vsan witness appliance download
In a vSAN cluster, each node has a copy of the data, and the Witness Appliance ensures that the data is consistent across all nodes. If a node fails or goes offline, the Witness Appliance helps to determine which node has the most up-to-date data and directs the other nodes to evacuate data to ensure availability. Once you have the ISO file, here is