VERITAS Storage Foundation 4.1 for Oracle RAC HP Serviceguard Storage Management Suite Extracts, December 2005

Overview: Storage Foundation
for Oracle
RAC
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This chapter describes the components of Storage Foundation for Oracle RAC and
interaction between these components.
What is RAC?
Real Application Clusters (RAC) is a parallel database environment that takes advantage
of the processing power of multiple computers. A cluster comprises two or more
computers, also known as nodes or servers. In RAC environments, all nodes concurrently
run Oracle instances and execute transactions against the same database. RAC
coordinates access to the shared data for each node to provide consistency and integrity.
Each node adds its processing power to the cluster as a whole and can increase overall
throughput or performance.
RAC serves a critical role in a robust solution for high availability. A properly configured
RAC environment can tolerate failures with minimal downtime and interruption to users.
If a node fails as clients access the same database on multiple nodes, clients attached to the
failed node can reconnect to a surviving node and resume access. Recovery after failure in
a RAC environment is far quicker than recovery for a failover database because another
instance is already up and running. The recovery process involves applying outstanding
redo log entries from the failed node.
RAC Architecture
From a high-level perspective, RAC involves multiple Oracle instances accessing a single
Oracle database and carrying out simultaneous transactions. An Oracle database is the
physical data stored in tablespaces on disk. An Oracle instance is a set of processes and
shared memory that provide access to the physical database. Specifically, the instance
involves server processes acting on behalf of clients to read data into shared memory and
make modifications to it, and background processes to write changed data to disk.
In traditional environments, only one instance accesses a database at a specific time.
Oracle RAC enhances scalability and availability by enabling multiple instances to access
the same database. This requires significant coordination between the instances to keep
each instance’s view of the data consistent.
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