Oracle Wait Interface: A Practical Guide to Performance Diagnostics & Tuning

What’s So Special About Waits in Real Application Clusters (RAC)?

Before we begin discussing wait events in a RAC environment, you need to understand how the buffer cache works in a RAC environment.

In a single database instance environment, there will be only one set of shared memory segments for the database. All I/O operations, SQL processing, and library cache operations are routed through one set of shared memory structures. In other words, the buffer cache and shared pool are local to that particular instance; at any point in time the processes attached to that particular instance will be accessing only one set of memory structures.

However, in the RAC environment, the scenario is entirely different. Multiple instances share the same database. ...