Any time a server process waits for an event to complete, it’s classified as a wait event.
The most common wait events are those caused by resource contention such as latch contention, buffer contention, and I/O contention.
Here’s a summary of the main wait classes in Oracle Database 11g:
• Administrative: Waits caused by administrative commands, such as rebuilding an index, for example.
• Application: Waits due to the application code.
• Cluster: Waits related to Real Application Cluster management.
• Commit: The single wait event log file sync, which is a wait caused by commits in the database.
• Concurrency: Waits for database resources that are used for locking, for example, latches.
• Idle: Waits that occur when a session isn’t active, for example, the 'SQL*Net message from client' wait event.
• Network: Waits incurred during network messaging.
• Other: Miscellaneous waits.
• Scheduler: Resource Manager–related waits.
• System I/O: Waits for background-process I/O, including the database writer background process’s wait for the db file parallel write event. Also included are archive-log–related
waits and redo log read-and-write waits.
• User I/O: Waits for user I/O. Includes the db file sequential read and db file scattered read events.
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