What is a Quorum Witness?
The HP 3PAR Quorum Witness is a virtual machine (VM) appliance deployed on ESXi that acts as an arbitrator in a split-brain scenario. You should always deploy the witness server on a third site with diverse routes to your two datacentres (DCs). As the witness is deployed to arbitrate in a split-brain scenario it needs to be hosted externally to your DCs.
The system essentially works by polling each 3PAR and allowing them to communicate back to the witness. In the event of a failure the witness will aid the decision making process of whether a 3PAR array should cease control of any volumes exported by it’s partner.