Architecture of AWR

Active Session History will be reviewed briefly before moving on to AWR snapshots.

Active Session History

The Active Session History (ASH) contains recent session activity. The memory assigned to the ASH comes from the SGA and is fixed for the instance lifetime. Its size also cannot exceed 5% of the shared pool size and takes up 2MB per CPU within these limits. 

The ASH works by sampling the v$session view every second and recordingwhat events the sessions are waiting for. The sampling process accesses the database internal structures directly. During the sampling process, inactive sessions are ignored. The ASH is designed as a rolling buffer in memory by over-writing older information as needed. The v$active_session_history view is used to access the ASH statistics. This view will contain one row for every active session per sample.