Database Health monitoring - Oracle

http://docs.oracle.com/cd/B28359_01/server.111/b28310/diag007.htm

Beginning with Release 11g, Oracle Database includes a framework called Health Monitor for running diagnostic checks on the database.

Health Monitor checks (also known as checkers, health checks, or checks) examine various layers and components of the database. Health checks detect file corruptions, physical and logical block corruptions, undo and redo corruptions, data dictionary corruptions, and more. The health checks generate reports of their findings and, in many cases, recommendations for resolving problems. Health checks can be run in two ways:

• Reactive—The fault diagnosability infrastructure can run health checks automatically in response to a critical error.
  
• Manual—As a DBA, you can manually run health checks using either the DBMS_HM PL/SQL package or the Enterprise Manager interface. You can run checkers on a regular basis if desired, or Oracle Support may ask you to run a checker while working with you on a service request.
  

Health Monitor checks store findings, recommendations, and other information in the Automatic Diagnostic Repository (ADR).
Health checks can run in two modes:

• DB-online mode means the check can be run while the database is open (that is, in OPEN mode or MOUNT mode).
  
• DB-offline mode means the check can be run when the instance is available but the database itself is closed (that is, in NOMOUNT mode).
  

All the health checks can be run in DB-online mode. Only the Redo Integrity Check and the DB Structure Integrity Check can be used in DB-offline mode.

Health monitor runs the following checks:

• DB Structure Integrity Check—This check verifies the integrity of database files and reports failures if these files are inaccessible, corrupt or inconsistent. If the database is in mount or open mode, this check examines the log files and data files listed in the control file. If the database is in NOMOUNT mode, only the control file is checked.
  
• Data Block Integrity Check—This check detects disk image block corruptions such as checksum failures, head/tail mismatch, and logical inconsistencies within the block. Most corruptions can be repaired using Block Media Recovery. Corrupted block information is also captured in the V$DATABASE_BLOCK_CORRUPTION view. This check does not detect inter-block or inter-segment corruption.
  
• Redo Integrity Check—This check scans the contents of the redo log for accessibility and corruption, as well as the archive logs, if available. The Redo Integrity Check reports failures such as archive log or redo corruption.
  
• Undo Segment Integrity Check—This check finds logical undo corruptions. After locating an undo corruption, this check uses PMON and SMON to try to recover the corrupted transaction. If this recovery fails, then Health Monitor stores information about the corruption in V$CORRUPT_XID_LIST. Most undo corruptions can be resolved by forcing a commit.
  
• Transaction Integrity Check—This check is identical to the Undo Segment Integrity Check except that it checks only one specific transaction.
  
• Dictionary Integrity Check—This check examines the integrity of core dictionary objects, such as tab$ and col$. It performs the following operations:
  
  • Verifies the contents of dictionary entries for each dictionary object.
    
  • Performs a cross-row level check, which verifies that logical constraints on rows in the dictionary are enforced.
    
  • Performs an object relationship check, which verifies that parent-child relationships between dictionary objects are enforced.
    

http://www.dba-oracle.com/oracle_database_health_check.htm