C.1 Classes of Wait Events

Every wait event belongs to a class of wait event.

The following list describes each of the wait classes.


Waits resulting from DBA commands that cause users to wait (for example, an index rebuild)


Waits resulting from user application code (for example, lock waits caused by row level locking or explicit lock commands)


Waits related to Oracle Real Application Clusters resources (for example, global cache resources such as 'gc cr block busy')


This wait class only comprises one wait event - wait for redo log write confirmation after a commit (that is, 'log file sync')


Waits for internal database resources (for example, latches)


Waits caused by inadequate configuration of database or instance resources (for example, undersized log file sizes, shared pool size)


Waits that signify the session is inactive, waiting for work (for example, 'SQL*Net message from client')


Waits related to network messaging (for example, 'SQL*Net more data to dblink')


Waits which should not typically occur on a system (for example, 'wait for EMON to spawn')


Contains events that signify delays in obtaining additional data in a pipelined environment. The time spent on these wait events indicates inefficiency or other problems in the pipeline. It affects features such as Oracle Streams, parallel queries, or DBMS_PIPE PL/SQL packages.


Resource Manager related waits (for example, 'resmgr: become active')

System I/O

Waits for background process I/O (for example, DBWR wait for 'db file parallel write')

User I/O

Waits for user I/O (for example 'db file sequential read')