Red Hat Summit26장. Configuring cluster quorum
In order to maintain cluster integrity and availability, cluster systems use a concept known as quorum to prevent data corruption and loss. A cluster has quorum when more than half of the cluster nodes are online. To mitigate the chance of data corruption due to failure, Pacemaker by default stops all resources if the cluster does not have quorum.
Quorum is established using a voting system. When a cluster node does not function as it should or loses communication with the rest of the cluster, the majority working nodes can vote to isolate and, if needed, fence the node for servicing.
For example, in a 6-node cluster, quorum is established when at least 4 cluster nodes are functioning. If the majority of nodes go offline or become unavailable, the cluster no longer has quorum and Pacemaker stops clustered services.
The quorum features in Pacemaker prevent what is also known as split-brain, a phenomenon where the cluster is separated from communication but each part continues working as separate clusters, potentially writing to the same data and possibly causing corruption or loss. For more information about what it means to be in a split-brain state, and on quorum concepts in general, see the Red Hat Knowledgebase article Exploring Concepts of RHEL High Availability Clusters - Quorum.
A Red Hat Enterprise Linux High Availability Add-On cluster uses the votequorum service, in conjunction with fencing, to avoid split brain situations. A number of votes is assigned to each system in the cluster, and cluster operations are allowed to proceed only when a majority of votes is present. The service must be loaded into all nodes or none; if it is loaded into a subset of cluster nodes, the results will be unpredictable. For information about the configuration and operation of the votequorum service, see the votequorum(5) man page on your system.
26.1. Configuring quorum options
There are some special features of quorum configuration that you can set when you create a cluster with the pcs cluster setup command. The following table summarizes these options.
| Option | Description |
|---|---|
|
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When enabled, the cluster can suffer up to 50% of the nodes failing at the same time, in a deterministic fashion. The cluster partition, or the set of nodes that are still in contact with the
The
The |
|
| When enabled, the cluster will be quorate for the first time only after all nodes have been visible at least once at the same time.
The
The |
|
|
When enabled, the cluster can dynamically recalculate |
|
|
The time, in milliseconds, to wait before recalculating |
For further information about configuring and using these options, see the votequorum(5) man page on your system.
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