VVR Terms Defined
This section defines the following VVR terms:
Primary and Secondary Host
The concept of Primary host and Secondary host are used only in the context of a particular Replicated Data Set (RDS), not per system. A system in the configuration can simultaneously be the Primary for one RDS and Secondary for others. This allows for very flexible replication configurations.
Replicated Data Set
VVR replicates data from a Primary host, where the application is running, to one or more Secondary hosts. A Replicated Volume Group (RVG) on the Primary host and its counterparts on the Secondary hosts make up an RDS. An RDS is not a Volume Manager object but just a concept used in VVR. An RDS enables grouping of the RVG on the Primary and its counterparts on the Secondaries.
Most VVR commands operate on an RDS, that is, the Primary RVG and all the Secondaries in the RDS. You can issue VVR commands from any host in an RDS unless otherwise noted. VVR performs the appropriate tasks on the required hosts in the RDS.
Local Host (localhost)
The host from which a command is issued is called the local host. The name of the Replicated Volume Group (RVG) on the local host represents the RDS. For example, to add a data volume to an RDS, issue the command from any host in the RDS, using the name of the RVG on that host to specify the RDS; VVR adds a data volume to the corresponding RVGs on all the hosts in the RDS.
Heartbeat Protocol
To ensure that the Secondary host can always detect communication loss regardless of update activity, the Primary host periodically sends a heartbeat message to the Secondary. If the Secondary misses a fixed number of heartbeat messages, it detects a communication loss and begins its recovery protocol. Likewise, if the Primary is unable to send a heartbeat message or if its heartbeat messages go unacknowledged, the RVG on the Primary host also detects a communication loss and enters its recovery procedure.
On successful completion of the recovery protocol, update activity resumes automatically unless some interim administrative command or error prevents it. Heartbeat messages use the UDP protocol. The choice of protocol is available for data and control messages.
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