Installation guide

Section B.1:Cluster Communication Mechanisms 171
B Supplementary Software Information
The information in the following sections can assist in the management of the cluster software con-
figuration.
B.1 Cluster Communication Mechanisms
A cluster uses several intra-cluster communication mechanisms to ensure data integrity and correct
cluster behavior when a failure occurs. The cluster uses these mechanisms to:
Control when a system can become a cluster member
Determine the state of the cluster systems
Control the behavior of the cluster when a failure occurs
The cluster communication mechanisms are as follows:
Quorum disk partitions
Periodically, each cluster system writes a timestamp and system status (
UP or DOWN) to the pri-
mary and backup quorum partitions, which are raw partitions located on shared storage. Each
cluster system reads the system status and timestamp that were written by the other cluster system
and determines if they are up to date. The cluster systems attempt to read the information from
the primary quorum partition. If this partition is corrupted, the cluster systems read the infor-
mation from the backup quorum partition and simultaneously repair the primary partition. Data
consistency is maintained through checksums and any inconsistencies between the partitions are
automatically corrected.
If a cluster system reboots but cannot write to both quorum partitions, the system will not be
allowed to join the cluster. In addition, if an existing cluster system can no longer write to both
partitions, it removes itself from the cluster by shutting down.
Remote power switch monitoring
Periodically, each cluster system monitors the health of the remote power switch connection, if any.
The cluster system uses this information to help determine the status of the other cluster system.
The complete failure of the power switch communication mechanism does not automatically result
in a failover.
Ethernet and serial heartbeats
The cluster systems are connected together by using point-to-point Ethernet and serial lines. Pe-
riodically, each cluster system issues heartbeats (pings) across these lines. The cluster uses this
information to help determine the status of the systems and to ensure correct cluster operation.