Redo log files store details of database changes and enable recovery from minor failures like power outages. They contain redo records with changes to data blocks. A database must have at least two redo log groups with each group containing one or more redo log members. The log writer process writes redo entries in the log buffer to online redo log files.

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