Red Hat Enterprise Linux 5 Cluster Suite Overview page 20

Chapter 1. Red Hat Cluster Suite Overview
apparent interruption to cluster clients. Cluster-service failover can occur if a cluster node fails or if a
cluster system administrator moves the service from one cluster node to another (for example, for a
planned outage of a cluster node).
To create a high-availability service, you must configure it in the cluster configuration file. A cluster
service comprises cluster resources. Cluster resources are building blocks that you create and
manage in the cluster configuration file — for example, an IP address, an application initialization
script, or a Red Hat GFS shared partition.
You can associate a cluster service with a failover domain. A failover domain is a subset of cluster
nodes that are eligible to run a particular cluster service (refer to
Note
Failover domains are not required for operation.
A cluster service can run on only one cluster node at a time to maintain data integrity. You can specify
failover priority in a failover domain. Specifying failover priority consists of assigning a priority level to
each node in a failover domain. The priority level determines the failover order — determining which
node that a cluster service should fail over to. If you do not specify failover priority, a cluster service
can fail over to any node in its failover domain. Also, you can specify if a cluster service is restricted
to run only on nodes of its associated failover domain. (When associated with an unrestricted failover
domain, a cluster service can start on any cluster node in the event no member of the failover domain
is available.)
Figure 1.9, "Failover
In
domain; therefore, Cluster Service X can only fail over between Node A and Node B. Failover Domain
2 is also configured to restrict failover with its domain; additionally, it is configured for failover priority.
Failover Domain 2 priority is configured with Node C as priority 1, Node B as priority 2, and Node D as
priority 3. If Node C fails, Cluster Service Y fails over to Node B next. If it cannot fail over to Node B, it
tries failing over to Node D. Failover Domain 3 is configured with no priority and no restrictions. If the
node that Cluster Service Z is running on fails, Cluster Service Z tries failing over to one of the nodes
in Failover Domain 3. However, if none of those nodes is available, Cluster Service Z can fail over to
any node in the cluster.
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Domains", Failover Domain 1 is configured to restrict failover within that
Figure 1.9, "Failover Domains").