Veritas Storage Foundation™ 5.0.1 for Oracle RAC Installation, Configuration, and Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

Figure 1-4 Cluster Communication
GAB Messaging
- Cluster Membership/State
Server
Server
NICNIC
NIC NIC
- Datafile Management
- File System Metadata
- Volume Management
Cluster Membership
At a high level, all nodes configured by the installer can operate as a cluster; these nodes form
a cluster membership. In SGeRAC, a cluster membership specifically refers to all systems
configured with the same cluster ID communicating by way of a redundant cluster interconnect.
Refer to Using Serviceguard Extension for RAC for additional information.
All nodes in a distributed system, such as SGeRAC, must remain constantly alert to the nodes
currently participating in the cluster. Nodes can leave or join the cluster at any time because of
shutting down, starting up, rebooting, powering off, or faulting processes. SGeRAC uses its
cluster membership capability to dynamically track the overall cluster topology.
SGeRAC uses LLT heartbeats to determine cluster membership:
• When systems no longer receive heartbeat messages from a peer for a predetermined interval,
a protocol excludes the peer from the current membership.
• GAB informs processes on the remaining nodes that the cluster membership has changed;
this action initiates recovery actions specific to each module. For example, CVM must initiate
volume recovery and CFS must perform a fast parallel file system check.
• When systems start receiving heartbeats from a peer outside of the current membership, a
protocol enables the peer to join the membership.
Cluster Communications
GAB provides reliable cluster communication between SGeRAC modules. GAB provides
guaranteed delivery of point-to-point messages and broadcast messages to all nodes. Point-to-point
messaging involves sending and acknowledging the message. Atomic-broadcast messaging
ensures all systems within the cluster receive all messages. If a failure occurs while transmitting
a broadcast message, GAB ensures all systems have the same information after recovery.
Low-level Communication: Port Relationship Between GAB and Processes
All components in SGeRAC use GAB for communication. Each process wanting to communicate
with a peer process on other nodes registers with GAB on a specific port. This registration enables
communication and notification of membership changes. For example, the SGeRAC engine (had)
registers on port h. had receives messages from peer had processes on port h. had also receives
notification when a node fails or when a peer process on port h becomes unregistered.
Some processes use multiple ports for specific communications requirements. For example, CVM
uses multiple ports to allow communications by kernel and user-level functions in CVM
independently.
Cluster Volume Manager
CVM is an extension of Veritas Volume Manager, the industry-standard storage virtualization
platform. CVM extends the concepts of VxVM across multiple nodes. Each node recognizes the
same logical volume layout, and more importantly, the same state of all volume resources.
CVM supports performance-enhancing capabilities, such as striping, mirroring, and mirror
break-off (snapshot) for off-host backup. You can use standard VxVM commands from one node
16 Introducing Serviceguard Extension for RAC