Q: How does VMware vSphere 5's FDM select a master?

A: The master/slave architecture vSphere 5’s Fault Domain Manager (FDM) uses for monitoring vSphere HA clusters uses an election process to determine which host is to be the master. This election process occurs any time the existing master fails, is shut down, or is placed into maintenance mode. It also occurs when vSphere HA is enabled, or when a management network partition occurs. The election process takes about 10 to 15 seconds.

The election process is defined by an algorithm with two published rules. For the first, the host with access to the greatest number of datastores wins. In the case of a tie, the second rule kicks in: The host with the lexically-highest Managed Object ID (MOID) is chosen. Care must be taken when attempting to rig this election because lexically here means, for example, that host-99 is in fact higher than host-100.

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