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. Oracle 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

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