READ ME before using the Veritas Storage Foundation™ 5.1 SP1 for Oracle RAC Administrator's Guide (April 2011)

Cluster Interconnect Communication Channel
The cluster interconnect provides the communication channel for all system-to-system communication,
in addition to one-node communication between modules. Low Latency Transport (LLT) and Group
Membership Services/Atomic Broadcast (GAB) make up the SG SMS Serviceguard for Oracle
RAC communications package central to the operation of SGeRAC.
Refer to Using Serviceguard Extension for RAC for additional information.
Low Latency Transport
LLT provides fast, kernel-to-kernel communications and monitors network connections. LLT functions
as a high performance replacement for the IP stack and runs directly on top of the Data Link Protocol
Interface (DLPI) layer. The use of LLT rather than IP removes latency and overhead associated with
the IP stack. The major functions of LLT are traffic distribution, heartbeats, and support for RAC
Inter-Process Communications (VCSIPC).
• Traffic Distribution
LLT distributes (load-balances) internode communication across all available cluster interconnect
links. All cluster communications are evenly distributed across as many as eight network links
for performance and fault resilience. If a link fails, LLT redirects traffic to the remaining links.
• Heartbeats
LLT is responsible for sending and receiving heartbeat traffic over network links. The Group
Membership Services function of GAB uses heartbeats to determine cluster membership.
• VCSIPC
RAC Inter-Process Communications (VCSIPC) uses the VCSIPC shared library for these
communications. VCSIPC leverages all features of LLT and uses LMX, an LLT multiplexer, to
provide fast data transfer between Oracle processes on different nodes.
Group Membership Services/Atomic Broadcast
The GAB protocol is responsible for cluster membership and cluster communications.
Figure 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.
Cluster Interconnect Communication Channel 11