vSAN 7 Update 1 now gives us high efficiency to this two node “Shared Witness” topology by allowing vSAN administrators to use a single shared witness for up to 64 two node vSAN clusters and hence reduces witness server/appliance footprint/resource usage in the central datacenter. The new UI for the witness appliance under Cluster->Monitor->vSAN->Two Node Clusters.
Two-Node vSAN ROBO cluster configuration was introduced some time back with vSAN 6.2 release couple of years back, this architecture was used primarily in large number of small branch offices, retail sites, IOT etc. All two node vSAN cluster configuration always needed a dedicated witness appliance for each of its deployment until vSAN 7. This means that there was a large footprint of witness appliances in the central site which hosts witnesses for all of its retail two node ROBO sites.
Certain Design and availability constraints might require multiple shared witness dedicated to a group of two node clusters, since losing a shared witness host will have a higher magnitude of vulnerability when it is server a large number of two node clusters and all the objects that shares the witness node will go to reduced availability state. It is recommended to deploy additional witness nodes in the centralized location to quickly replace the witness if the primary witness server fails so that all objects can be re-protected immediately by replacing the witness server. Shared witness appliance/server will require additional memory resources ~12GB for up to 30 two node clusters and needs additional if the size grows higher.
Configure Shared witness
To configure a “shared witness” we need to make sure that the witness server which is deployed has a minimum memory allocation of 12GB if not the witness VM can be powered-off, increase the memory allocation and power it back on. vSAN Engineering team is planning to include options to change the compute resources for shared witness during witness-OVA/OVF deployment.
In the following Example (screenshots) the witness appliance is already configured for ROBO-1 Cluster, the same witness server will be configured to serve as witness node for cluster : ROBO-2.
That concludes this article on Shared Witness on vSAN 7 Update 1 , make sure to read my other blogs on vSAN 7 U1 feature walk-through/guides. If you liked this blog feel free to share with your colleagues and friends.