1. Redo Logs and Recovery John Hibbard Senior Principal Instructor
Minneapolis MN
Revised by Dr. Philip Cannata
University of Texas, Austin, Texas

2. Redo Buffer Operation User #1 UPDATE emp SET sal = 10 WHERE id = 1234
When user enters…
User #1
data file 2
undo file 3
data file 4
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack
database buffer cache

3. Redo Buffer Operation User #1 UPDATE emp SET sal = 10 WHERE id = 1234
When user enters…
User #1
data file 2
undo file 3
data file 4
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there)
database buffer cache

4. Redo Buffer Operation User #1 UPDATE emp SET sal = 10 WHERE id = 1234
When user enters…
User #1
data file 2
undo file 3
data file 4
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there) U1
The undo block is then retrieved from disk to the database buffer cache (unless it’s already there)
database buffer cache

5. Redo Buffer Operation User #1 UPDATE emp SET sal = 10 WHERE id = 1234
When user enters…
User #1
data file 2
undo file 3
data file 4
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there) U1
The undo block is then retrieved from disk to the database buffer cache (unless it’s already there) 5
The undo data is created showing the ‘before image’
database buffer cache

6. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T
The data block is retrieved from disk to the database buffer cache (unless it’s already there) U1
The udo block is then retrieved from disk to the database buffer cache (unless it’s already there) 5
The undo data is created showing the ‘before image’
And an entry recording this change is made in the REDO buffer
database buffer cache

7. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T
Information is also recorded identifying the location of the data segment being changed. U1 5
database buffer cache

8. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T
Then the data block is changed... x 10 U1 5
database buffer cache

9. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
Then the data block is changed... x 10
And an entry recording this change is made in the REDO buffer U1 5
database buffer cache

10. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
Then the data block is changed... x 10
And an entry recording this change is made in the REDO buffer U1 5
database buffer cache

11. Redo Buffer Operation Keypoint Redo Buffer
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T
Keypoint T
Transaction identifier
Column address
Value of the column that changed
Only the very basic data is stored in the redo. The main redo items are:

12. Redo Buffer Operation Redo Buffer This continues for all changes
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
This continues for all changes x 10
User # 2 U1
UPDATE prod
SET large_col... WHERE id = XYZ 5
database buffer cache

13. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
This continues for all changes x 10
User # 2 U1
UPDATE prod
SET large_col... WHERE id = XYZ 5
XYZ
Zack
database buffer cache

14. Redo Buffer Operation Redo Buffer User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
T Zack
This continues for all changes x 10
User # 2 U1
UPDATE prod
SET large_col... WHERE id = XYZ 5 U2
XYZ
Zack
Zack
database buffer cache

15. Redo Buffer Operation Redo Buffer xx User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
T Zack
This continues for all changes
T &&& x 10
User # 2 U1
UPDATE prod
SET large_col... WHERE id = XYZ 5 U2
XYZ xx
&&&&
Zack
Zack
database buffer cache

16. Redo Buffer Operation Redo Buffer xx User #1
When user enters…
Redo Buffer
User #1
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
UPDATE emp SET sal = 10 WHERE id = 1234
XYZ
Zack T T
T Zack
This continues for all changes
T &&&& x 20 x 10 T
User # 2 U1 T
UPDATE prod
SET large_col... WHERE id = XYZ 5 10
User #1 U2
XYZ
UPDATE emp SET sal = 20 WHERE id =1234 xx
&&&&
Zack
Zack
database buffer cache

17. Redo Buffer Operation Redo Buffer xx User #1 commit;
Then a user commits
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
User #1
XYZ
Zack T T
commit;
T Zack
file:block
file:block
T &&&& x 20 x 10
2:123 T U1 T 5
3:12 10 U2
XYZ
3:65 xx
&&&&
4:89
Zack
Zack
database buffer cache

18. Redo Buffer Operation Redo Buffer xx User #1 commit;
Then a user commits
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
User #1
XYZ
Zack T T
commit;
T Zack
And an entry recording the commit is made in the REDO buffer
T &&&& x 20 x 10 T U1 T 5 10
T1 commit SCN time stamp U2
XYZ xx
&&&&
Zack
Zack
database buffer cache

19. Redo Buffer Operation Redo Buffer xx User #1 commit;
Then a user commits
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
User #1
XYZ
Zack T T
commit;
T Zack
And an entry recording the commit is made in the REDO buffer
T &&&& x 20 x 10 T U1
This entry consists of: T 5
a “code” to indicate this is a commit (as opposed to a rollback or checkpoint)
a timestamp
the current SCN 10
T1 commit SCN time stamp U2
XYZ xx
&&&&
Zack
Zack
database buffer cache

20. Thought you’d never ask!!!
Redo Buffer Operation
What is an SCN?
Thought you’d never ask!!!

21. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint

22. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint
For Example:
3256
this SCN was assigned to the last transaction that issued a commit

23. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint
For Example:
3256
3257
this SCN was assigned to the last transaction that issued a commit

24. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint
For Example:
3256
3257
3258
this SCN was assigned to the last transaction that issued a commit

25. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint
For Example:
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
this SCN was assigned to the last transaction that issued a commit

26. System Change Number SCN is the System Change Number
sequentially assigned by LGWR
to indicate the “time” of a commit, rollback or checkpoint
For Example:
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
If one SCN is larger than another it means it was committed after the smaller was committed…
and vice versa

27. Redo Buffer Operation Redo Buffer
The log data will now get written to the logfile by the log writer (LGWR).
Redo Buffer T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
LGWR writes when:
Commit
Buffer is 1/3 full
Checkpoint
1 Meg of data in buffer
DBWR writes
3 seconds
Log 1
Log 2

28. Redo Buffer Operation Redo Buffer LGWR
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
Notice that even though it was only transaction T1 that committed the data for T87 also was written even though it is not committed.
Log 1
Log 2

29. Redo Buffer Operation Redo Buffer
While this data is being written to the log file other data can continue to be placed in the redo buffer until the buffer is full. T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
T &&&&
T #### T T
T2 commit SCN# time stamp
Log 1
Log 2

30. Redo Buffer Operation Redo Buffer
While this data is being written to the log file other data can continue to be placed in the redo buffer until the buffer is full.
Notice the two commits. T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
Log 1
Log 2

31. Redo Buffer Operation Redo Buffer
While this data is being written to the log file other data can continue to be placed in the redo buffer until the buffer is full T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
Since LGWR is busy the two commits are “piggybacked”
LGWR will write both at the same time and will use the same SCN for both
Log 1
Log 2

32. Redo Buffer Operation Redo Buffer
When LGWR finishes writing the group of log records the user is notified that their data has been committed.
LGWR then begins writing the next group of log records. T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
LGWR
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
Log 1
Log 2

33. Redo Buffer Operation Redo Buffer LGWR Log 1 Log 2 T1 3 12 - - 5
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
LGWR
Log 1
Log 2

34. Redo Buffer Operation Redo Buffer
Additional data continues to be added to the redo buffer overwriting the previously written data T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
LGWR
T #### T T T T T T T
Log 1
Log 2

35. Redo Buffer Operation Redo Buffer
Additional data continues to be added to the redo buffer overwriting the previously written data T T T T T T T T
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
LGWR
T #### T T T T T T T
Log 1
Log 2

36. OUCH! Redo Buffer Operation Redo Buffer
The REDO BUFFER just filled up! T T T T T T T T
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
LGWR
T #### T T T T T T T
Log 1
Log 2

37. Redo Buffer Operation
All the users doing DML get this object on their screen for their efforts!

38. Redo Buffer Operation

39. REDO BUFFER needs to be larger
Redo Buffer Operation
Redo Buffer
Solution:
REDO BUFFER needs to be larger T T T T T T T T
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
LGWR
T #### T T T T T T T
Log 1
Log 2

40. NOW…. To revert back to our discussion about the physical redo logs

41. Redo Log Operation
LGWR continues writing to the log until the log fills up.
LGWR
Log 1
Log 2

42. Redo Log Operation
LGWR continues writing to the log until the log fills up.
LGWR
Log 1
Log 2

43. Redo Log Operation
LGWR continues writing to the log until the log fills up.
LGWR
Log 1
Log 2

44. Redo Log Operation This is called a log switch.
LGWR continues writing to the log until the log fills up.
LGWR then begins writing to the next redo log.
This is called a log switch.
LGWR
Log 1
Log 2

45. Redo Log Operation
For safety’s sake redo logs should always be mirrored.
ALTER DATABASE ADD LOGFILE MEMBER log1b TO GROUP 1;
ALTER DATABASE ADD LOGFILE MEMBER log2b TO GROUP 2;
Note: this method is recommended instead of or in addition to using hardware mirroring
Log 1
Log 2 a
Log 1
Log 2 b

46. Redo Log Operation
For safety’s sake redo logs should always be mirrored.
ALTER DATABASE ADD LOGFILE MEMBER log1b TO GROUP 1;
ALTER DATABASE ADD LOGFILE MEMBER log2b TO GROUP 2;
Note: this method is recommended instead of or in addition to using hardware mirroring
LGWR will then write to the multiple logs simultaneously
LGWR
Log 1
Log 2 a
Log 1
Log 2 b

47. Database Recovery
The Oracle recovery process has three parts performed in this order:
Roll Forward: all changes made to data blocks that had not been written to the data files on disk are performed again by reapplying (redoing) the images originally recorded on the Redo Log.
Open Database: users can begin normal operations
Rollback: uncommitted transactions are rolled back using the undo log

48. Database Recovery Redo Log T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
When recovery begins the control file is accessed to determine which enty in the redo log is to be used to start the roll foward portion of recovery.
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
Control File
database buffer cache

49. Database Recovery Redo Log T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
That entry in the Redo Log is accessed.
T &&&& T
The referenced block is retrieved from disk to the database buffer cache (unless it’s already there). It happens to be a roll back block. U1 T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
database buffer cache

50. Database Recovery Redo Log T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
That entry in the Redo Log is accessed.
T &&&& T
The referenced block is retrieved from disk to the database buffer cache (unless it’s already there). It happens to be a roll back block. U1 T 5
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
The roll back data is recreated showing the ‘before image’
database buffer cache

51. Database Recovery Redo Log T1 3 12 - - 5 Then… T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T
Then… T
T Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there)
T &&&& T U1 T 5
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
database buffer cache

52. Database Recovery Redo Log T1 3 12 - - 5 Then… T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T
Then… T
T Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there)
T &&&& T U1 T 5
Notice that the value in the data block is greater than the redo data!
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
database buffer cache

53. Database Recovery Redo Log T1 3 12 - - 5 Then… T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T
Then… T
T Zack
The data block is retrieved from disk to the database buffer cache (unless it’s already there) x 10
T &&&& T U1 T 5
Notice that the value in the data block is greater than the redo data!
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
The data is recreated showing the ‘after image’
database buffer cache

54. Database Recovery Redo Log T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
Then the next entry .... x 10
T &&&& T U1 T 5
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T U2
Zack
database buffer cache

55. Database Recovery Redo Log xx T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
and the next x 10
T &&&& T
XYZ U1 T xx
&&&&
Zack 5
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T U2
Zack
database buffer cache

56. Database Recovery Redo Log xx T1 3 12 - - 5 T1 2 123 41 6 10
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
etc., x 10
T &&&& T
XYZ U1 T xx
&&&&
Zack 5 10
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T U2
Zack
database buffer cache

57. Database Recovery Redo Log xx
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
etc., etc., x 20
T &&&& T
XYZ U1 T xx
&&&&
Zack 5
Notice that the value in the data block has now been restored to the value on disk. 10
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T U2
Zack
database buffer cache

58. Database Recovery Redo Log xx C C
data file 2
undo file 3
data file 4
Tran File Block Row Column Value id
XYZ
Zack T T
T Zack
etc., etc., etc. x 20
T &&&& C T
XYZ U1
The transaction has been “re-committed” T xx
&&&&
Zack 5 10 C
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T U2
Zack
database buffer cache

59. Database Recovery Keypoint Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
data file 2
undo file 3
data file 4
XYZ
Zack
Keypoint
After processing the remainder of the redo log.... x 20 C
XYZ U1 xx
&&&&
Zack 5
the buffer cache and data files will have been rebuilt to appear as they were at the time of the crash. 10 C U2
Zack
database buffer cache

60. Database Recovery Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
data file 2
undo file 3
data file 4
XYZ
Zack
The database is then opened so the users can begin work..... x 20 C
XYZ U1 xx
&&&&
Zack 5 10 C U2
Zack
database buffer cache

61. Database Recovery Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
data file 2
undo file 3
data file 4
then all uncommitted transactions will be rolled back.
XYZ
Zack
The database is then opened so the users can begin work..... x 20 C
XYZ U1 xx
&&&&
Zack 5 10 C U2
Zack
database buffer cache

62. Database Recovery Beginning with 9i Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
Beginning with 9i
The method for performing the roll forward phase has been changed to improve performance.
DBWn 3 / 12
2/123
3 / 65
4 / 89

63. Database Recovery Beginning with 9i Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
Beginning with 9i
An entry is placed in the redo log whenever DBWR writes data to disk .
It identifies the blocks that were just written.
DBWn 3 / 12
2/123
3 / 65
4 / 89

64. Database Recovery Beginning with 9i Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
Beginning with 9i
To improve performance during instance recovery the redo log is processed twice
first to identify the entries that need to be processed
second to process the selected entries
DBWn 3 / 12
2/123
3 / 65
4 / 89

65. Identify the Entries First pass of redo data Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
First pass of redo data
Identify redo that needs to be processed
No data blocks are read
Very fast
DBWn 3 / 12
2/123
3 / 65
4 / 89

66. Identify the Entries Copy the REDO....
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Copy the REDO....

67. Remove entries for those blocks already written....
Identify the Entries
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Remove entries for those blocks already written....

68. Remove entries for those blocks already written....
Identify the Entries
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Remove entries for those blocks already written....

69. Remove entries for those blocks already written....
Identify the Entries
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Remove entries for those blocks already written....

70. Remove entries for those blocks already written....
Identify the Entries
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Remove entries for those blocks already written....

71. Remove entries for those blocks already written....
Identify the Entries
Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Remove entries for those blocks already written....

72. Identify the Entries Finish the list.... Redo Log
Tran File Block Row Column Value T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89
Finish the list....

73. Identify the Entries Redo Log Tran File Block Row Column Value
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89 T T T T T T T T T T T T T

74. Identify the Entries Redo Log Tran File Block Row Column Value
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T
DBWn 3 / 12
2/123
3 / 65
4 / 89 T T T T T T T T T T T T T

75. Second pass of redo data
Process the Entries
Second pass of redo data
Recover using these selected entries T T
T Zack
T &&&& T T
T1 commit SCN# time stamp
T &&&&
T #### T T
T2 commit SCN# time stamp T T
T3 commit SCN# time stamp
T #### T T T T T T T T T T T T T T T

76. This concludes the demonstration of
Redo Logs and Recovery