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diff --git a/doc/features/server-quorum.md b/doc/features/server-quorum.md new file mode 100644 index 00000000000..7b20084cea8 --- /dev/null +++ b/doc/features/server-quorum.md @@ -0,0 +1,44 @@ +# Server Quorum + +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. + +## Options + +Server quorum is controlled by two parameters: + + * **cluster.server-quorum-type** + + This value may be "server" to indicate that server quorum is enabled, or + "none" to mean it's disabled. + + * **cluster.server-quorum-ratio** + + 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. + +## Best Practices + +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. + + + |