Active Session History

Since ASH data is of high volume, flushing all ASH data is not advisable. Therefore, the data is filtered out by writing it to disk using MMON every 30 minutes and by MMNL (Memory Monitor Light) when the buffer gets full.

AWR Snapshots

The Automatic Workload Repository (AWR) is a collection of persistent system statistics stored in the system defined Workload Repository (WR) schema. The WR schema resides in the SYSAUX tablespace and is one of the main SYSAUX components.

A snapshot is a set of performance statistics captured at a certain time in the database. It is used for computing the rate of change of a statistic. Every snapshot is identified by a snapshot sequence number (snap_id) that is unique within AWR. Snapshots are generated every 60 minutes by default. The snapshot interval parameter can be used to change this frequency. This interval determines if snapshots of all major activities in the database are being obtained. Automatic snapshots are generated using an internal MMON task, which runs periodically.  Manual snapshots can be taken by using the database control or a PL/SQL procedure.