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.
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
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.