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RAC extensions

Oracle RAC relies on several support services provided by VCS. Key features include Veritas Cluster Server Membership Manager (VCSMM) and Cluster Inter-Process Communication (VCSIPC), and LLT Multiplexer (LMX).

Veritas Cluster Server membership manager

To protect data integrity by coordinating locking between RAC instances, Oracle must know which instances actively access a database. Oracle provides an API called skgxn (system kernel generic interface node membership) to obtain information on membership. SF Oracle RAC implements this API as a library linked to Oracle after you install Oracle RAC. Oracle uses the linked skgxn library to make ioctl calls to VCSMM, which in turn obtains membership information for clusters and instances by communicating with GAB on port o.

Veritas Cluster Server inter-process communication

To coordinate access to a single database by multiple instances, Oracle uses extensive communications between nodes and instances. Oracle uses Inter-Process Communications (VCSIPC) for Global Enqueue Service locking traffic and Global Cache Service cache fusion. SF Oracle RAC uses LLT to support VCSIPC in a cluster and leverages its high-performance and fault-resilient capabilities.

Oracle has an API for VCSIPC, System Kernel Generic Interface Inter-Process Communications (skgxp), that isolates Oracle from the underlying transport mechanism. As Oracle conducts communication between processes, it does not need to know how data moves between systems; the cluster implementer can create the highest performance for internode communications without Oracle reconfiguration.

LLT multiplexer

Oracle instances use the skgxp library for interprocess communication. This interface enables Oracle to send communications between processes on instances.

SF Oracle RAC provides a library dynamically linked to Oracle at installation time to implement the skgxp functionality. This module communicates with the LLT Multiplexer (LMX) using ioctl calls.

The LMX module is a kernel module designed to receive communications from the skgxp module and pass them on to the correct process on the correct instance on other nodes. The LMX module "multiplexes" communications between multiple processes on other nodes. LMX leverages all features of LLT, including load balancing and fault resilience.