VMware Cluster Master Election: Goodness Levels

Did you know that every 5.x VMware ESXi cluster has a master server to handle cluster functions. Of course you did. But do you know know how to find out what role a server is currently hosting, and how to manage those roles?

As you know, the addition of vCenter 5.1 added a few . . . well . . . major things: Single Sign-on or SSO, the Web Client, a plethora of new interface views, and so forth. One such new view is pictured below.

vSphere Web Client Cluster Summary View

Navigate to the vSphere Cluster | Monitor | VMware HA tab and you will see in the Summary section an entry that tells you the cluster master currently elected (all other cluster members are then by default slaves) and if any cluster members are out of contact with the master for any reason. But what determines which server is the cluster master?

When vSphere HA is enabled for a cluster, all active hosts (those not in standby or maintenance mode, or not disconnected) participate in an election to choose the cluster's master host. The host that mounts the greatest number of datastores has an advantage in the election. Only one master host typically exists per cluster and all other hosts are slave hosts. If the master host fails, is shut down or put in standby mode, or isremoved from the cluster a new election is held. — vSphere Availability Guide

So, let's say you have a mid-size to large environment, or really anywhere you are working with multiple clusters. Maybe you want to ensure that standard operations will have the first host in any cluster as the master, to help you trouble any HA issues that may arise—as they will inevitably do. To do this turn on your cluster functionality and ensure that the first host has at least one more datastore than the rest, and it will be given preference for master election.

If you would like to force a certain host to always be elected the master anytime cluster HA operations are reconfigured, you can also accomplish this by configuring Advanced Options on the ESXi host in question. (Use with extreme caution)

VMware HA Cluster Settings | Advanced Options

The option fdm.nodeGoodness specifies the "goodness" of a node, the idea being the node with the greatest goodness level is elected as master. 

When a master election is held, the FDMs exchange a goodness value, and the FDM with the largest goodness value is elected master. Ties are broken using the host IDs assigned by vCenter Server. This parameter can be used to override the computed goodness value for a given FDM. To force a specific host to be elected master each time an election is held and the host is active, set this option to a large positive value. This option should not be specified at the cluster level. — VMware KB 2033250