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1. Oracle9i Database Administrator: Implementation and Administration 1
Chapter 5
The Redo Log Files

2. Oracle9i Database Administrator: Implementation and Administration 2
Objectives
 Learn to describe redo log files,
groups, and members
 Manage redo log groups and
members

3. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 4
Oracle9i Architecture: A Typical Server

5. Oracle9i Database Administrator: Implementation and Administration 5
Introduction to Online
Redo Log Files
Components:
•Redo log group
•Online redo log
file or member
•Archived redo
log file
•ARCn

6. Oracle9i Database Administrator: Implementation and Administration 6
Introduction to Online
Redo Log Files
Components:
•Redo log buffer
•LGWR
•CKPT

7. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 13
Managing Redo Log Files
The COMMIT
command tells
the database to
log the
changes to the
redo log file

14. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 19
Renaming or Moving
a Redo Log File
 Must be done while the group is not
ACTIVE
 Steps:
1. Shut down the database:
SHUTDOWN IMMEDIATE
2. Rename or relocate files using the operating
system
3. Mount the database
STARTUP MOUNT

20. Oracle9i Database Administrator: Implementation and Administration 20
Renaming or Moving
a Redo Log File
 Steps (continued):
4. Inform database of changed file name:
ALTER DATABASE
RENAME FILE '<X:xxx><oldfilename1>',
'<X:xxx><oldfilename2>'
TO '<X:xxx><newfilename1>',
'<X:xxx><newfilename2>';
5. Open the database:
ALTER DATABASE OPEN;

21. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 23
Dropping Redo Log Members or
Groups
Steps to drop redo log member:
1. Query V$LOG to confirm that group is
inactive
2. Drop redo log member:
ALTER DATABASE DROP LOGFILE MEMBER
'<X:xxx><filename>';
3. Delete associated file in the operating
system

24. Oracle9i Database Administrator: Implementation and Administration 24
Dropping Redo Log Members or
Groups
Steps to drop redo log group:
1. Query V$LOG to confirm that group is
inactive: SELECT * FROM V$LOG;
2. Drop redo log group:
ALTER DATABASE DROP LOGFILE GROUP <n>;
3. Delete associated file(s) in the operating
system

25. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 27
Finding Redo Log Information in
Data Dictionary Views
States of a redo log group:
 UNUSED
 CURRENT
 ACTIVE
 CLEARING
 CLEARING_CURRENT
 INACTIVE

28. Oracle9i Database Administrator: Implementation and Administration 28
Viewing Control File Data

29. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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. Oracle9i Database Administrator: Implementation and Administration 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