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Redo Log Files

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Transcript

  • 1.
    • Chapter 5
    • The Redo Log Files
  • 2. Objectives
    • Learn to describe redo log files, groups, and members
    • Manage redo log groups and members
  • 3. Introduction to Online Redo Log Files
    • Online redo log files store details as the database is used including:
    • Checkpoints
    • Data Changes (DML)
    • Structural changes (DDL)
    • Data file changes
  • 4. Oracle9 i Architecture : A Typical Server
  • 5. Introduction to Online Redo Log Files
    • Components:
    • Redo log group
    • Online redo log
    • file or member
    • Archived redo
    • log file
    • ARCn
  • 6. Introduction to Online Redo Log Files
    • Components:
    • Redo log buffer
    • LGWR
    • CKPT
  • 7. Introduction to Online Redo Log Files File 1 is written to File A by the ARCn process after the log switch is complete
  • 8. Introduction to Online Redo Log Files File 1 and File 3 are written to by the LGWR process simultaneously until both are full
  • 9. The Purpose of Redo Log Files
    • Redo log files aid in recovery from short term data loss such as brief power outage:
    • Store changes that may not be written to the data files yet
    • Store checkpoints to allow recovery to resynchronize data files with correct changes
    • Cannot completely handle recovery from major loss such as an entire data file
  • 10. The Purpose of Redo Log Files
    • Recovery procedures (overview):
    • Prior to failure:
      • Perform full database backup regularly
      • Run database in ARCHIVELOG mode
    • After failure:
      • Restore from full backup
      • Apply archived redo log files (oldest to newest)
      • Apply online redo log file
  • 11. The Structure of Redo Log Files
    • Redo log buffer is flushed to the redo log file when:
      • A transaction COMMITS
      • The redo log buffer becomes 1/3 full
      • The redo log buffer contains >1 M of updated records
      • A checkpoint occurs
  • 12. The Structure of Redo Log Files
    • Redo log file components:
    • Redo record (also called redo entry)
      • Relates to one data block
      • Made up of one or more change vectors
  • 13. Managing Redo Log Files The COMMIT command tells the database to log the changes to the redo log file
  • 14. Log Switches and Checkpoints
    • To manually signal a log switch:
      • ALTER SYSTEM SWITCH LOGFILE;
    • A log switch triggers a checkpoint
    • Checkpoint is a signal to write all dirty buffers to the appropriate files
    • Checkpoint increments the System Change Number (SCN)
  • 15. Log Switches and Checkpoints
    • How the SCN is used:
      • SCN is recorded in the redo log buffer
      • SCN is recorded in the header of each data file that was written to
      • During recovery, SCN of each datafile is compared to the SCN in the redo log file
        • If a match, the data file is up to date
        • If redo log file has higher SCN, changes are reapplied to the data file from the redo log file
  • 16. Multiplexing and Other Maintenance
    • Multiplexed redo log files requires multiple members in each group
    • A log group continues to function as long as it has at least one good member
    • If all members of the current group become damaged, the database forces a log switch
    • The database shuts down immediately if a log switch fails
  • 17. Adding a Member to a Group
    • Add members to groups while the database is running
    • Do not specify SIZE because new members are automatically assigned the same size as other members in the group
    • Command syntax:
    • ALTER DATABASE ADD LOGFILE MEMBER '<X:xxx><filename>'
    • TO GROUP <n>;
  • 18. Adding a New Group
    • Add a new group while the database is running
    • Specify SIZE and group number
    • List all members to be created with the group (must be at least one)
    • Command syntax:
      • ALTER DATABASE ADD LOGFILE GROUP <n>
      • ( '<X:xxx><filename>', '<X:xxx><filename>')
      • SIZE <n>;
  • 19. Renaming or Moving a Redo Log File
    • Must be done while the group is not ACTIVE
    • Steps:
      • Shut down the database:
        • SHUTDOWN IMMEDIATE
      • Rename or relocate files using the operating system
      • Mount the database
        • STARTUP MOUNT
  • 20. Renaming or Moving a Redo Log File
    • Steps (continued):
      • Inform database of changed file name:
        • ALTER DATABASE
        • RENAME FILE '<X:xxx><oldfilename1>',
        • '<X:xxx><oldfilename2>'
        • TO '<X:xxx><newfilename1>',
        • '<X:xxx><newfilename2>';
      • Open the database:
        • ALTER DATABASE OPEN;
  • 21. Dropping Redo Log Members or Groups
    • Reasons for dropping members or groups:
    • Bad disk, so must recreate member
    • Tuning recommendation calls for reducing members or groups
    • Corrupted members can be deleted and replaced later
  • 22. Dropping Redo Log Members or Groups
    • Rules when dropping redo log members:
      • Each group must have at least one member
      • The group affected must not be ACTIVE and (if in ARCHIVELOG mode) must be archived
    • Rule when dropping groups:
      • Database requires at least two groups
      • The group affected must not be ACTIVE and (if in ARCHIVELOG mode) must be archived
  • 23. Dropping Redo Log Members or Groups
    • Steps to drop redo log member:
      • Query V$LOG to confirm that group is inactive
      • Drop redo log member:
        • ALTER DATABASE DROP LOGFILE MEMBER
        • '<X:xxx><filename>';
      • Delete associated file in the operating system
  • 24. Dropping Redo Log Members or Groups
    • Steps to drop redo log group:
      • Query V$LOG to confirm that group is inactive: SELECT * FROM V$LOG;
      • Drop redo log group:
        • ALTER DATABASE DROP LOGFILE GROUP <n>;
      • Delete associated file(s) in the operating system
  • 25. Dropping Redo Log Members or Groups
    • Additional notes:
    • If group is ACTIVE, force log switch:
      • ALTER SYSTEM SWITCH LOGFILE;
    • If group still active, force checkpoint:
      • ALTER SYSTEM CHECKPOINT;
    • An alternative to dropping a corrupt group and recreating the members:
      • ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <n>;
  • 26. Archiving a Redo Log Group
    • Advantages of archiving redo log groups:
      • Point-in-time recovery to a point earlier than online redo log files contain
      • The ability to query archived log files with LogMiner
      • The ability to set up and maintain a standby database
  • 27. Finding Redo Log Information in Data Dictionary Views
    • States of a redo log group:
      • UNUSED
      • CURRENT
      • ACTIVE
      • CLEARING
      • CLEARING_CURRENT
      • INACTIVE
  • 28. Viewing Control File Data
  • 29. Chapter Summary
    • Redo log files primarily contain information on database changes
    • A database has at least two redo log groups
    • A redo log group has at least one member
    • Redo log files support automatic recovery from minor failures
    • A redo log group with multiple files is multiplexed
  • 30. Chapter Summary
    • The SCN is incremented at a checkpoint
    • Maintenance you can do with redo log files and groups:
      • Add a new file (member) to a group
      • Add a new group
      • Rename or relocate a member
      • Drop a member
      • Drop a group
      • Clear a group
  • 31. Chapter Summary
    • Placing a database in ARCHIVELOG mode causes redo log files to be archived after a log switch
    • Many initialization parameters set the behavior of the archiving function
    • The V$LOG and V$LOGFILES dynamic performance views display information about redo log groups and members