HP StorageWorks XP Cluster Extension Software Administrator Guide (T1656-96035, April 2010)

RHCS cluster setup considerations
Quorum
In RHCS, the quorum is based on a simple voting majority of the defined nodes in a cluster. To re-form
successfully, a majority of all possible votes is required.
Each cluster node is assigned a number of votes, and they contribute to the cluster while they are
members. If the cluster has a majority of all possible votes, it has quorum (also called quorate);
otherwise, it does not have quorum.
Fencing
Cluster software adjusts the node membership based on various failure scenarios. The concept of
quorum defines which set of nodes continue to define the cluster. To protect data, nodes that do not
have quorum are removed from the cluster. The non-quorate nodes that are removed must be prevented
from accessing the shared resources. This process is called fencing.
HP iLO fencing is one method that can be used with RHCS to restrict cluster node access to shared
resources.
Observe the following guidelines when using HP iLO network configurations with RHCS clusters:
• HP iLO can be connected to the client access network or to a different network, but the network
must be routable.
• HP iLO should not be on the network that is used for cluster communication.
• The HP iLO of each cluster system must be accessible over the network from every other cluster
system.
To handle infrequent failures of the HP iLO fencing (such as a switch failure), you can set up a backup
fence method for redundancy.
HP iLO fencing can be used on HP Proliant systems with built-in iLO hardware. For third-party systems,
other power control fencing methods can be used.
NOTE:
IPMI fencing can be used for Integrity servers that do not support RIBCL scripting.
Qdisk configuration
Red Hat recommends the use of a Qdisk configuration to bolster quorum to handle failures such as
half (or more) of the members failing, a tie-breaker in equal split partition, and a SAN failure.
In an XP Cluster Extension configuration with multiple storage arrays, a Qdisk configuration is not
supported.
Failover domains
A cluster service is associated with a failover domain, which is a subset of cluster nodes that are
eligible to run a particular cluster service. To maintain data integrity, each cluster service can run on
only one cluster node at a time. By assigning a cluster service to a restricted failover domain, you
can limit the nodes that are eligible to run a cluster service in the event of a failover, and you can
order the nodes by preference to ensure that a particular node runs the cluster service (as long as that
node is active).
XP Cluster Extension features18