REF: Planning Exchange 2010 DAG - Part 5 - Witness Server

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Important!!
If the witness server you specify isn't an Exchange 2010 server, you must add the Exchange Trusted Subsystem universal security group to the local Administrators group on the witness server. These security permissions are necessary to ensure that Exchange can create a directory and share on the witness server as needed. If the proper permissions aren't configured, the following error is returned:
Error: An error occurred during discovery of the database availability group topology. Error: An error occurred while attempting a cluster operation. Error: Cluster API "AddClusterNode() (MaxPercentage=12) failed with 0x80070005. Error: Access is denied."



Witness Server Requirements
A witness server is a server outside of a DAG that's used to achieve and maintain quorum when the DAG has an even number of members. DAGs with an odd number of members do not use a witness server. All DAGs with an even number of members will use a witness server. The witness server can be any computer running Windows Server. There is no requirement that the version of the Windows Server operating system of the witness server match the operating system used by the DAG members.
Quorum is maintained at the cluster level, underneath the DAG. A DAG has quorum when the majority of its members are online and can communicate with the other online members of the DAG. This notion of quorum is one aspect of the concept of quorum in Windows failover clustering. A related and necessary aspect to quorum in failover clusters is the quorum resource. The quorum resource is a resource inside a failover cluster that provides a means for arbitration leading to cluster state and membership decisions. The quorum resource also provides persistent storage for storing configuration information. A companion to the quorum resource is the quorum log, which is a configuration database for the cluster. The quorum log contains information such as which servers are members of the cluster, what resources are installed in the cluster, and the state of those resources (for example, online or offline).
It is critical that each DAG member have a consistent view of how the DAG's underlying cluster is configured. The quorum acts as the definitive repository for all configuration information relating to the cluster. The quorum is also used as a tie-breaker to avoid “split-brain” syndrome. Split brain syndrome is a condition that occurs when DAG members cannot communicate with each other but are up and running. Split brain syndrome is prevented by always requiring a majority of the DAG members (and in the case of DAGs with an even number of member, the DAG witness server) to be available and interacting for the DAG to be operational.

Blog Extended Reading


More Information & Reference
1. Planning for High Availability and Site Resilience



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