Oracle9i Database Administrator: Implementation and Administration Chapter 5 The Redo Log Files Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration Objectives Learn to describe redo log files, groups, and members Manage redo log groups and members Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration Oracle9i Architecture: A Typical Server Oracle9i Database Administrator: Implementation and Administration

Introduction to Online Redo Log Files Components: Redo log group Online redo log file or member Archived redo log file ARCn Oracle9i Database Administrator: Implementation and Administration

Introduction to Online Redo Log Files Components: Redo log buffer LGWR CKPT Oracle9i Database Administrator: Implementation and Administration

Introduction to Online Redo Log Files File 1 is written to File A by the ARCn process after the log switch is complete Oracle9i Database Administrator: Implementation and Administration

Introduction to Online Redo Log Files File 1 and File 3 are written to by the LGWR process simultaneously until both are full Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

Managing Redo Log Files The COMMIT command tells the database to log the changes to the redo log file Oracle9i Database Administrator: Implementation and Administration

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) Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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>; Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration 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>; Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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; Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

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>; Oracle9i Database Administrator: Implementation and Administration

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 Oracle9i Database Administrator: Implementation and Administration

Finding Redo Log Information in Data Dictionary Views States of a redo log group: UNUSED CURRENT ACTIVE CLEARING CLEARING_CURRENT INACTIVE Oracle9i Database Administrator: Implementation and Administration

Viewing Control File Data Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration 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 Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration 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 Oracle9i Database Administrator: Implementation and Administration

Oracle9i Database Administrator: Implementation and Administration 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 Oracle9i Database Administrator: Implementation and Administration