How to use the Automatic Database Diagnostic Monitor(一)

Oracle9i and before， DBA may access Oracle performance statistics by using tools such as STATSPACK；
In Oracle 10g, the database by default takes Automatic Workload Repository (AWR) snapshots 。

1. AWR(Automatic Workload Repository)
    AWR data is flushed from memory to disk using internal kernel calls and a dedicated background process (MMON). 
    Data collecting doesn't require manual configuration, like Statspack snapshots in previous releases.
    AWR provides automatic purging of data (7 days by default) 
    AWR snapshots can be preserved so that they are not purged
    EXECUTE dbms_workload_repository.create_snapshot();

2.ASH (Active Session History) 
    collects samples of active sessions every second

3.ADDM(Automatic Database Diagnostic Monitor)
   ADDM is a powerful self-diagnostic engine ,ADDM is invoked automatically every time that a new AWR snapshot is generated.
   Manually generate a new ADDM report:
   $ORACLE_HOME/rdbms/admin/addmrpt.sql

4.Use AWR rather than statspack
   because:
   The repository is created and maintained automatically 
   The report can be generated in text or html mode 
   The report displays additional information regarding wait classes, metrics, OS stats, etc
   To generate an AWR report :
   $ORACLE_HOME/rdbms/admin/awrrpt.sql

參考文獻：
   1.Note 276103.1- PERFORMANCE TUNING USING 10g ADVISORS AND MANAGEABILITY FEATURES
  2.Note 250655.1 - How to use the Automatic Database Diagnostic Monitor
  3.