vSAN 7 Update 1 introduces a shared witness option for 2-node cluster configurations. The ability to use the same witness host/virtual appliance for multiple 2-node clusters is cost-efficient in terms of physical resources, manageability, design, and operations. It also saves you valuable time from not having to deploy and setup multiple witness hosts for each remote 2-node cluster.
VMware vSAN 2-node architecture has always been a perfect solution for organizations that have many small branch offices or retail sites. It is also very beneficial for small businesses and startups who want to avoid the significant up-front costs associated with storage hardware. The shared witness host appliance additionally reduces the amount of physical resources needed at the central site, resulting in a greater level of savings for а large number of 2-node branch office deployments. For example, a topology with 30 remote sites would have previously needed 240 GB of RAM at the central site for the witness host appliance, while using the shared witness model would require 16 GB. One single shared witness can be shared across a maximum of 64 2-node clusters, supporting up to 64, 000 components and requires at least 6 CPUs, and 32GB memory allocation for the Witness. Additionally, we support a max of 1000 components per 2-node cluster. Build-in checks will monitor if the maximum number of components for a witness in a 2-node cluster has been reached and they would not allow the witness to be converted into a shared witness.
The beauty of this new feature is that most of the well-known requirements for a witness host appliance remain unchanged for the shared witness appliances. Namely, shared witnesses can be deployed in the same topology scenarios where a standard witness is deployed (e.g. running in a VCPP location), the shared witness can be either a virtual witness host appliance or a physical host. The New UI will allow you to manage your shared witness and monitor its relations to all 2-node clusters.

There are also a couple of conditions to be met before you can enable the shared host appliance architecture:
• As of vSAN 7 U1, the witness node should be upgraded first (to maintain backward compatibility). Upgrading of the witness can be accomplished through VUM but is not supported by vLCM at this time.

• All vSAN clusters participating in a shared witness must use the new on-disk format (ODF) version associated with vSAN 7 U1 (v13)
The updated witness host appliance OVA will feature a new “extra-large witness deployment” that will be sized to handle the increased cluster and component counts. Compatibility checks are also provided to monitor the network and the version consistency across the clusters served by a single shared witness.
It is recommended to install a standby witness and fast changeover in the event of a failure of the primary witness. The importance of this deployment augments in the case of shared witness since the witness is shared among a larger number of clusters.
The shared witness host appliance is the vSAN 7 U1 approach to a better experience for your remote 2-node clusters deployment and management. It saves time, resources, and reduces complexity.
Here’s are two demos showing how easy it is to manage your shared witness within the UI.