1. 7202ICT – Database Administration
Lecture Three
Database Architecture
DATABASE WRITER (DBWR) PROCESS
UPDATE OPERATION
REDO LOG BUFFER
DATABASE COMMITS
REDO LOG FILES
CHECK POINTS
ARCH PROCESS
TRANSACTIONS OVERVIEW
Reading Oracle Concept Manuel Chapter 8

2. Database writer process
SGA
Database Buffer Cache
Shared Pool
Data
Files
Server
User
DBWR

3. DATABASE WRITER (DBWR) PROCESS
DBWR manages the database buffer cache so that user processes can always find free buffers as needed.(picture)
The DBWR Process
Writes all changed (modified) data buffers to the appropriate data files
Uses an LRU algorithm to keep the most recently used data blocks in memory while aging out the least recently used blocks
NB: You can start more than one DBWR process. This would be required if one DBWR process is incapable of handling the demands of the database.

4. DATABASE WRITER (DBWR) PROCESS
The DBWR process will wake up and begin writing dirty buffers to disk when:
The dirty list reaches a pre-defined threshold (set using database parameters DB_BLOCK_WRITE_BATCH)
A server process scans a specified number of buffers (set using database parameters DB_BLOCK_MAX_SCAN) in the LRU list without finding a free buffer (server process then signals DBWR process)
A time-out occurs (these occur about every 3 seconds)
A checkpoint occurs

5. LOG TRANSACTIONS
Oracle will record all changes made to the database in the redo log buffer. A background process called the LOG WRITER (LGWR) will write the information recorded in the redo log buffer to disk. Another background process called the ARCHIVER (ARCH) may optionally be used to archive all redo information.(Picture)
The redo log buffer is a circular buffer containing information about changes made to the database. This information is stored in redo entries.

6. LOG TRANSACTIONS Redo Entries
Stores all changes made to the database in the redo entries of the redo log buffer.
Used to reconstruct or rollback changes made to the database when database recovery is necessary due to a database crash.
NB: ORACLE does not consider a transaction to be complete until the LGWR has written the appropriate redo logs for that transaction to disk. It is this action that determines the success of a commit statement.

7. Logwriter Process SGA Data Server User DBWR LGWR ARCH Redo Log Buffer
Shared SQL Area
Shared Pool
SGA
Database Buffer Cache
Redo Log
Buffer
Server
User
DBWR
LGWR
Data
Files
ARCH

8. LOGWRITER PROCESS (LGWR)
The Log Writer (LGWR) process writes redo log entries to disk.(Picture)
LGWR writes redo log buffer entries to the redo log files when:
A Commit occurs
The redo log buffer pool reaches one third full threshold
The DBWR needs to clean the buffer blocks for a checkpoint
A time-out occurs

9. LOGWRITER PROCESS (LGWR)
N.B
There is only one redo log writer per instance
A commit confirmation is not issued to the user until the transaction is recorded in the log file
LGWR cannot write if the ARCH process is active on the group that LGWR needs to write to. In this case all database actions shall queue will until the ARCH process is completed.
The user must commit a transaction to make the changes permanent in the database.

10. DATABASE COMMITS Commit Operation A user issues the commit command
A commit record is placed in the redo log buffer
LGWR flushes the redo log buffers to current log file
The user is notified that the transaction has been committed
Resource locks are released on data and rollback blocks
The data block is marked as “pinned” (clean but out of synch with disk)
DBWR will eventually write database block to disk

11. REDO LOG FILES
Redo log files record all changes made to the database, and are used for data recovery. If the redo log files are mirrored (multiplexed), then the same redo log information is written to multiple online redo log files.(Picture)
Redo Log Files
Redo log files are written to in a circular fashion
There must be at least 2 redo log groups
Mirrored Redo Log Files
Group members are updated simultaneously
Mirrored redo log files protect against loss of a redo log file

12. REDO LOG FILES

13. Log Switches
A log switch is when LGWR switches from one redo log file to another
A log switch occurs when LGWR has filled on log file group
A log switch can be forced by the DBA if current redo file needs to be archived
A log switch occurs when the database is shut down
A checkpoint automatically occurs at a log switch

14. CHECKPOINTS
During a checkpoint, DBWR writes all dirty buffers in the database buffer cache to disk, guaranteeing that all data blocks modified since the previous checkpoint are actually written to disk.
Checkpoints Occur
At every log switch
A specified number of seconds (LOG_CHECKPOINT_TIMEOUT) after the last checkpoint
When a predetermined number of redo log blocks have been written to disk since the last checkpoint
At instance shutdown
When forced by a DBA
When a tablespace is taken off-line (ie: taking a table off-line)

15. CHECKPOINTS
During a checkpoint and after the associated log switch, LGWR will update the headers of database and control files, unless the optional checkpoint (CKPT) process has been started, in which case it will perform this task.
The parameter LOG_CHECKPOINT_TIMEOUT is used to determine the interval of time between checkpoints.
The parameter LOG_CHECKPOINT_INTERVAL is used to determine how many redo file blocks must be filled since the previous checkpoint, before another checkpoint is initiated.
Database checkpoints ensure that all modified database buffers are written to the database files. The database files are marked current as of a given time and the checkpoint is recorded in the control file.(Picture)

16. Checkpoints CKPT Process
Require that instance recovery need only apply changes that have been made since the last checkpoint
Allow an on-line redo log file to be reused by guaranteeing that all changes stored in the redo log file are also written to the appropriate data file
Checkpoints do not halt activity on the database, nor are current transactions affected.
CKPT Process
Updates headers of data and control files after a checkpoint has been completed
More frequent checkpoints will reduce the time necessary to recover a database from instance failure, but will degrade overall performance (tradeoff)

17. Checkpoints SGA CKPT Server User DBWR LGWR ARCH Shared Pool
Shared SQL Area
Shared Pool
SGA
Database Buffer Cache
Redo Log
Buffer
Server
User
DBWR
LGWR
Data
Files
ARCH
CKPT
Control Files

18. ARCH PROCESS
The Archiver Process (ARCH) is an optional process, which when enabled, will copy online redo log files to a designated storage area or device once they become full.(Picture)
Archiver Process
Redo log files are copied to tape or disk for media failure recovery (damaged disk)
ARCH operates only when a log file group switch occurs
ARCH is optional and is present only when automatic archiving is enabled, or when manually requested
ARCH may write to a tape device or to a disk

19. Savepoints
mark an interim point within a transaction to which a transaction can later be rolled back.
Syntax – SAVEPOINT
SAVEPOINT savepoint_name
Syntax – ROLLBACK
ROLLBACK TO savepoint_name

20. Savepoint Features
Useful during interactive trial and error situations
Useful within structured programming where:
Main program calls sub-routine
Sub-routine sets savepoint
Sub-routine processes statements
Sub-routine encounters error and rolls back to savepoint
Sub-routine returns control to main program and reports error
Maximum number of savepoints per transaction is determined by SAVEPOINTS parameter
Rolling back to a savepoint`
Rolls back changes since savepoint_name
Release locks acquired since savepoint_name