Installation guide

Chapter 2. Cluster Management with CMAN
Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation for
cluster manager) performs cluster management in the High Availability Add-On for Red Hat Enterprise
Linux. CMAN is a distributed cluster manager and runs in each cluster node; cluster management is
distributed across all nodes in the cluster.
CMAN keeps track of membership by monitoring messages from other cluster nodes. When cluster
membership changes, the cluster manager notifies the other infrastructure components, which then
take appropriate action. If a cluster node does not transmit a message within a prescribed amount of
time, the cluster manager removes the node from the cluster and communicates to other cluster
infrastructure components that the node is not a member. Other cluster infrastructure components
determine what actions to take upon notification that node is no longer a cluster member. For
example, Fencing would disconnect the node that is no longer a member.
CMAN keeps track of cluster quorum by monitoring the count of cluster nodes. If more than half the
nodes are active, the cluster has quorum. If half the nodes (or fewer) are active, the cluster does not
have quorum, and all cluster activity is stopped. Cluster quorum prevents the occurrence of a "split-
brain" condition — a condition where two instances of the same cluster are running. A split-brain
condition would allow each cluster instance to access cluster resources without knowledge of the
other cluster instance, resulting in corrupted cluster integrity.
2.1. Clust er Quorum
Quorum is a voting algorithm used by CMAN.
A cluster can only function correctly if there is general agreement between the members regarding
their status. We say a cluster has quorum if a majority of nodes are alive, communicating, and agree
on the active cluster members. For example, in a thirteen-node cluster, quorum is only reached if
seven or more nodes are communicating. If the seventh node dies, the cluster loses quorum and can
no longer function.
A cluster must maintain quorum to prevent split-brain issues. If quorum was not enforced, quorum, a
communication error on that same thirteen-node cluster may cause a situation where six nodes are
operating on the shared storage, while another six nodes are also operating on it, independently.
Because of the communication error, the two partial-clusters would overwrite areas of the disk and
corrupt the file system. With quorum rules enforced, only one of the partial clusters can use the
shared storage, thus protecting data integrity.
Quorum doesn't prevent split-brain situations, but it does decide who is dominant and allowed to
function in the cluster. Should split-brain occur, quorum prevents more than one cluster group from
doing anything.
Quorum is determined by communication of messages among cluster nodes via Ethernet. Optionally,
quorum can be determined by a combination of communicating messages via Ethernet and through a
quorum disk. For quorum via Ethernet, quorum consists of a simple majority (50% of the nodes + 1
extra). When configuring a quorum disk, quorum consists of user-specified conditions.
Note
By default, each node has one quorum vote. Optionally, you can configure each node to have
more than one vote.
Red Hat Ent erprise Linux 6 High Availabilit y Ad d- O n O verview
10