Server quorum is a feature intended to reduce the occurrence of "split brain" after a brick failure or network partition. Split brain happens when different sets of servers are allowed to process different sets of writes, leaving data in a state that can not be reconciled automatically. The key to avoiding split brain is to ensure that there can be only one set of servers - a quorum - that can continue handling writes. Server quorum does this by the brutal but effective means of forcing down all brick daemons on cluster nodes that can no longer reach enough of their peers to form a majority. Because there can only be one majority, there can be only one set of bricks remaining, and thus split brain can not occur.
Server quorum is controlled by two parameters:
This value may be "server" to indicate that server quorum is enabled, or "none" to mean it's disabled.
This is the percentage of cluster nodes that must be up to maintain quorum. More precisely, this percentage of nodes plus one must be up.
Note that these are cluster-wide flags. All volumes served by the cluster will be affected. Once these values are set, quorum actions - starting or stopping brick daemons in response to node or network events - will be automatic.
If a cluster with an even number of nodes is split exactly down the middle, neither half can have quorum (which requires more than half of the total). This is particularly important when N=2, in which case the loss of either node leads to loss of quorum. Therefore, it is highly advisable to ensure that the cluster size is three or greater. The "extra" node in this case need not have any bricks or serve any data. It need only be present to preserve the notion of a quorum majority less than the entire cluster membership, allowing the cluster to survive the loss of a single node without losing quorum.