doc: add documentation for server-quorum feature

Change-Id: Ifaf5c18a320270a9b98c7b25536281fa42b64ad3
BUG: 1086765
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/7492
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>

1 files changed, 41 insertions, 0 deletions

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+# 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.