Oracle RAC Wait Events

Analyzing and interpreting what causes sessions to wait is an important method to determine where time is spent. In Oracle RAC, the wait time is attributed to an event which reflects the exact outcome of a request. For example, when a session on an instance is looking for a block in the global cache, it does not know whether it will receive the data cached by another instance or whether it will receive a message to read from disk. The wait events for the global cache convey precise information and waiting for global cache blocks or messages is:

  • Summarized in a broader category called Cluster Wait Class

  • Temporarily represented by a placeholder event which is active while waiting for a block, for example:

    • gc current block request

    • gc cr block request

  • Attributed to precise events when the outcome of the request is known, for example:

    • gc current block 3-way

    • gc current block busy

    • gc cr block grant 2-way

In summary, the wait events for Oracle RAC convey information valuable for performance analysis. They are used in Automatic Database Diagnostic Monitor (ADDM) to enable precise diagnostics of the effect of cache fusion.