Archived Redo log File

 Archived Redo Log File :

• Filled online redo log files can be archived.
• There are two advantages in running the database in ARCIHIVELOG mode and archiving online redo log files :

- Recovery: A Database backup togather with online and archived redo log files can guarantee recovery of all committed transaction.

- Backup: This can be performed while the database is open

• By Default, The database is created in NOARCHIVELOG mode. 

One of the important decisions that a database administrator has to make is whether the database is configured to operate in ARCHIVELOG mode or in NOARCHIVELOG mode.

NOARCHIVELOG MODE :

In noarchivelog mode, the online redo log files are overwritten each time an online redo log file is filled, and log switches occur. LGWR does not overwrite an online redo log file group until the checkpoint for that group is completed.

ARCHIVELOG MODE :

If the database is configured to run in ARCHIVELOG mode, inactive groups of filled online redo log files must be archived. Because all changes made to the database are recorded in the online redo log files,  The Database administrator should take the physical backup of the archived online redo log file. Backup of the archived redo log file is used to recover the database without losing any committed data when Database recovery required.

There are two ways  in which online redo log files can be archived :

• Manually
• Automatically ( recommended method )

The LOG_ARCHIVE_START initialization parameter indicates whether archiving should be automatic or manual when the instance starts up.

• TRUE: TRUE indicates that archiving is automatic. ARCn initiates the archiving of the filled log group at every log switch.
• FALSE: The default value is FALSE, and it indicates that the DBA archives the filled online redo log files manually. The DBA must manually execute a command each time you want to archive an online redo log file. All or specific online redo log files can be archived manually.

Please review some important facts about Archive redo log files :

• Archiving accomplished automatically by ARCn
• Archiving accomplished manually through SQL statements
• When the online redo log file successfully archived below event occurs :

- An entry in the control file is made.

- Records: archive log name, log sequemce number, and high and low system change number (SCN)

• Filled online redo log files cannot be reused until: 

- A checkpoint has taken place

- File has been archived by ARCn

• Can be multiplexed
• Maintained by the DBA

Information about archive logs can be obtained from V$INSTANCE view.

SQL> SELECT archiver FROM V$INSTANCE;

ARCHIVE

-------

STARTED

The archiver parameter having a different status as below.

STOPPED: This status shows that the archiving has stopped working.

STARTED: This status shows that archiving is in progress and archiving is done by the Oracle Background Process ARCn.

FAILED: This status shows that the archiver failed to archive a log last time but will try again within 5 minutes.

Thank you !!

Managing Online Redo Log Files With OMF.

Managing Online Redo Log File With OMF :

• Define the DB_CREATE_ONLINE_LOG_DEST_n parameter :

DB_CREATE_ONLINE_LOG_DEST_1

DB_CREATE_ONLINE_LOG_DEST_2

• A group can be added with no file specification.

ALTER DATABASE ADD LOGFILE;

• Dropping a Group

ALTER DATABASE DROP LOGFILE GROUP 3;

Define the DB_CREATE_ONLINE_LOG_DEST_n  parameter: 

To create online redo log files to be managed by OMF, the DB_CREATE_ONLILNE_LOG_DEST_n  parameter must be defined. The parameter must be set for each multiplexed copy identified by the n value.

In the example above two groups have been created with 2 members each. The names will be generated automatically (such as  "ora_1_wo94n2xi.log") and displayed in the alertSID.log. The default size is 100 MB.

Adding Log File Group:

To create a new group of online redo log files, the DBA uses the ALTER DATABASE ADD LOGFILE command. The command has been modified so that the file specification is not necessary.

In the example above, To add a log file with 2 members, One in the location defined by the 

DB_CREATE_ONLINE_LOG_DEST_1  and one in 

DB _CREATE_ONLINE_LOG_DEST_2.

Unique filenames for the logfile members have generated automatically and displayed in the alertSID.log. the Default Size is 100MB.

Dropping a Group :

The above example drops the log file Group 3, and its operating system files associated with each OMF log file members in Group 3.

Archived Redo Log Files and OMF :

Archived redo log files cannot be OMF.

Obtaining Group and Member Information:

Information about a group and its members can be obtained by querying the following views. 

• V$LOG
• V$LOGFILE

V$LOG  view :

The following query returns information about the online redo logfile from the control file :

SQL> SELECT group#, sequence#, bytes, members, status FROM v$log;

    GROUP#  SEQUENCE#      BYTES    MEMBERS STATUS

-------- ---------- ---------- ---------- ----------------

       1     403637  209715200          2 CURRENT

       2     403634  209715200          2 INACTIVE

       3     403635  209715200          2 INACTIVE

       4     403636  209715200          2 INACTIVE

       5     403632  209715200          2 INACTIVE

       6     403633  209715200          2 INACTIVE

6 rows selected.

The following items are the most common values for the Status column : 

• UNUSED: This status indicates that the online redo log file group has never been written to. This is the state of the online redo log file group that was just added.
• CURRENT: This status indicates the current online redo log file group. This implies that the online redo log file group is active.
• ACTIVE: This status indicates that the online redo log file group is active but it is not the current online redo log file group. It is needed for crash recovery. It may be in use for block recovery. It may or may not be archived.
• CLEARING: This status indicates that the log is being re-created as an empty log after an ALTER DATABASE CLEAR LOGFILE command. After the log is cleared, the status changes the UNUSED.
• CLEARING_CURRENT: This status indicates that the current log file is being cleared of a closed thread. The log can stay in this status id there is some failure in the switch, such as an input/output (I/O) error writing the new log header.
• INACTIVE: This status indicates that the online redo log file group is no longer needed for instance recovery. It may or may not be archived.

V$LOGFILE  View :

To obtain the names of all the members of a group, query the V$LOGFILE view.

SQL> SELECT * FROM V$LOGFILE;

GROUP# STATUS  TYPE    MEMBER        IS_RECOVERY_DEST_FILE

------- ------- ------- ----------------- ---------------

2            ONLINE  /u01/oradata/ORCL/log02a.rdo       NO

2            ONLINE  /u01/oradata/ORCL/log02b.rdo       NO

1            ONLINE  /u01/oradata/ORCL/log01a.rdo       NO

1            ONLINE  /u01/oradata/ORCL/log01b.rdo       NO

4 rows selected.

 The value of the STATUS column could be one of the following.

• INVALID: This status indicates that the file is inaccessible.
• STALE: This status indicates that the contents of the file are incomplete.
• DELETED: This status indicates that the file is no longer used.
• Blank indicates that the file is in use. 

Thank You !!

Adding,Dropping, Clearing and Relocating or Renaming Online Redo Log Files and Redo log File Members

Adding Online Redo Log File Groups: 

 

Generally, we all use the below command to add Redo Log File Groups. Suppose If you have 2 redo log groups in your Environment and Due to the heavy transaction, Database performance is affecting, So in this scenario, you can add an online redo log file groups to run Your database smoothly and Ensure your Database availability and reduce the burden of the oracle process.

Command Used for Online Redo log file Group Creation :

SQL> ALTER DATABSE ADD LOGFILE GROUP 3

('$HOME/ORADATA/u01/log3a.log',

'$HOME/OARADATA/u02/log3b.log') SIZE 1M; 

In some cases, you might need to create additional log file groups. For example, Adding groups can solve availability problems. To create a new group of online redo log files, Use the above SQL command and use the path as per your requirement. 

Redo Log File Group creation syntax is as below:

SQL> ALTER DATABASE ADD LOGFILE [GROUP  integer]  filespec

[, [GROUP  integer]  Filespec ] ... SIZE ];

You Specify the Name and Location of the members with the file specification. The Value of the GROUP parameter can be selected for each online redo log filegroup. If you omit this parameter, the Oracle Server Generates its Value Automatically. 

Adding Online Redo Log File Members :

We use below SQL command to add Online Redo Log File Group Members. You can add new members to existing Online Redo Log File Groups using the following Command. 

SQL> ALTER DATABASE ADD LOGFILE MEMBER 

'$HOME/ORADATA/u04/log1c.log'  To  GROUP 1,

'$HOME/ORADATA/u04/log2c.log ' To  GROUP 2,

'$HOME/ORADATA/u04/log3c.log'  To  GROUP 3 ;

Use the Fully specified name of the log file members, Otherwise, the files are created in a default directory of the database server.

If the file already exists, it must have the same size, and you must specify the REUSE  option. You can Identify the target group either by specifying one or more members of the group or by specifying the group numbers.

Dropping Online Redo Log File Groups :

Generally, We use the Below command to Drop an online redo log file group :

SQL> ALTER DATABASE DROP LOGILE GROUP 3;

To increase or decrease the size of the online redo log file groups, Add a new online redo log file group (With the new size) and then drop the old one.

An entire online redo log file group can be dropped with the following ALTER DATABASE DROP LOGFILE  command. Syntax of Command is as below:

ALTER DATABSE DROP LOGFILE {GROUP  interger | ('filename',filename')};

 

Restrictions

• An instance requires at least two groups of online redo log files.
• An active or Current group Can not be dropped.
• When an online redo log file Group is dropped, the operating system files are not deleted.

Dropping Online Redo Log File Members :

We use the below command to drop an online redo log file members:

SQL>ALTER DATABASE DROP LOGFILE MEMBER 

'$HOME/ORADATA/u04/log3c.log';

You may want to drop an online redo log file member because it is invalid. Use the following  ALTER DATABASE DROP LOGFILE MEMBER command if you want to drip one or more specific online redo log file members.

Command Syntax :

ALTER DATABASE DROP LOGFILE MEMBER 'filename' ;

Restrictions:

• If the member you want to drop is the last valid member of the group, you can not drop that member.
• If the group is current, you must force a log switch before you can drop the member.
• If the Database is running in ARCHIVELOG mode and the log file group to which the member belongs is not archived, then the member can not be dropped.
• When an online redo log file member is dropped, the operating system file is not deleted if you not using the OMF feature.

Relocating or Renaming the Online Redo Log Files :

Relocate or Rename online redo log files by one of the two following ways :

• ALTER DATABASE RENAME FILE command

-  Shut Down the Database.

-  Copy the online redo log files to the new Location.

-  Place the database in MOUNT mode. 

-  Execute the Command .

SQL> ALTER DATABSE RENAME FILE 

'$HOME/ORADATA/u01/log2a.log' To '$HOME/ORADATA/u02/log1c.log' ;

- Open Databae for normal operation.

• Add new members and drop old members.

The Location of Online redo log files can be changed by renaming the online redo log files. Before renaming the online redo log files, Ensure that the new online redo log file exists. The Oracle server changes only the pointers in the control files but does not physically rename or create any operating system files.

Steps for Relocating or Renaming Online Redo Log Members:

1. Shut down the database 

SQL>  SHUTDOWN IMMEDIATE

2. Copy the online redo log files to the new location.

3. Startup the database in mount mode, but do not open it.

 SQL>  CONNECT / AS SYSDBA

 SQL> STARTUP MOUNT

4. Rename the online redo log members using ALTER DATABASE RENAME FILE command.

SQL> ALTER DATABASE RENAME FILE 

'$HOME/ORADATA/u01/log2a.log' To '$HOME/ORADATA/u02/log1c.log' ;

 5. Open the Database for normal operation.

SQL> ALTER DATABASE OPEN;

 

Clearing Online Redo Log Files:

For Clearing the online redo log files by using the following ways :

• ALTER DATABASE CLEAR LOGFILE command can be used to reinitialize an online redo log file.

SQL> ALTER DATABASE CLEAR LOGFILE GROUP 2 ;

• Use the UNARCHIVED keyword to avoid archiving the corrupted online redo log file.

SQL> ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 2 ;

 

An online redo log file might become corrupted while the database is open, and ultimately stop database activity because archiving cannot continue. In this Situation, the ALTER DATABASE CLEAR LOGFILE command can be used to reinitialize the online redo log file without shutting down the database.

This command can overcome two situations where dropping online redo log files is not possible :

• If there are only two log groups.
• The corrupt online redo log file belongs to the current group

If the corrupt online redo log file has not been archived, use the UNARCHIVED keyword in the command to clear the corrupted online redo log files and avoids archiving them. The cleared online redo log files are available for use even though they were not archived.

If you clear a log file that is needed for recovery of a backup, then you can no longer recover from that backup. Oracle Writes a message in the alert log describing the backups from which you can not recover.  So we should have to perform a fresh backup of the Database if this kind of situation comes.

 

Online Redo Log File Configuration:

To determine the appropriate number of online redo log files for a database instance, you must test different configurations.

In some cases, a database instance may require only two groups. In another situation, a database instance may require additional groups to guarantee that the groups are always available to LGWR. 

For example: if the messages in the LGWR trace file or in the alert log file indicate the LGWR frequently must wait for a group because a checkpoint has not completed or a group has not been archived, you should add groups.

Location of Online Redo Log File:

When you multiplex the online redo log files, Place members of a group on different disks. By doing this, even if one member is not available but other members are available, the instance does not shut down.

Separate archive log files and online redo log files on different disks to reduce contention between the ARCn and LGWR background processes.

Datafiles and online redo log files should be placed on different disks to reduce LGWR and DBWn contention and reduce the risk of losing both data files and online redo log files in the event of media failure.     

Sizing Online Redo Log Files :

The minimum size of an online redo log file is 50 KB, and the maximum size is specific to the operating system. Members of the different groups can have different size, however, there is no benefit in having different-sized groups.

Different Sized groups should be required as a temporary result only if you want to change the size of the members of the online redo log file groups. In this case, you should create a new online redo log file  groups with different sizes, and then remove the old groups

The following situations might influence the configuration of the online redo log files: 

• Number of  log switches and checkpoints
• Number and amount of redo entries
• Amount of space on the storage medium. for example on tape if archiving is enabled. 

Thank you !!    

Online Redo Log Files

Online Redo Log Files:

Online Redo log files have the following characteristics : 

• Record all changes made to data
• Provide a Recovery mechanism
• Can be organized into groups
• At least two groups required 

Online redo log files provide the way to redo transactions in the event of a database failure. Every transaction is written synchronously to the Redo Log Buffer, then gets flushed to the online redo log files in order to provide a recovery mechanism in case of media failure. (With Exceptions such as direct load inserts in objects with the NOLOGGING clause enabled.)

This includes transactions that have not yet been committed, Undo segment information, and schema and object management statements. Online redo log files are used in a situation such as an instance failure to recover committed data that has not been written to the data files. The online redo log files are used only for recovery.

 

Structure of the Online Redo Log Files:

                          Online Redo Log File Structure 

The DBA can set up the oracle database to maintain copies of online redo log files to avoid losing database information due to a single point of failure.

Online Redo Log File Groups :

• A set of identical copies of online redo log files is called an online redo log file group.
• The LGWR background process concurrently writes the same information to all online redo log files in a group.
• The Oracle Server needs a minimum of two online redo log file groups for the normal operation of a  database.

Online Redo Log File Members :

• Each online redo log file in a group is called a member.
• Each member in a group has identical sequence numbers and are of the same size. The log sequence number is assigned each time that the Oracle Server writes to a log group to uniquely identify each online redo log file. 
• The current log sequence number is stored in the control file and in the header of all the data files.  

Creating Initial Online Redo Log Files :

The Initial set of online redo log file groups and members are created during the database creation.

The following parameters limit the number of the online redo log file : 

• The MAXLOGFILES parameter in the CREATE DATABASE command specifies the absolute maximum of online redo log file groups.
• The maximum and the default value for MAXLOGFILES are dependent on your operating system.
• The MAXLOGMEMBERS parameter used in the CREATE DATABASE command determines the maximum number of members per group. The Maximum and default value for MAXLOGMEMBERS is dependent on your operating system.
  

How Online Redo Log Files Work:

• Online Redo Log  Files are used in a cyclic fashion.
• When an online redo log file is full, LGWR (background Process ) will move to the next log group.

- Called a log Switch

- Checkpoint operations also occure

- Information Written to the control file

The Oracle Server sequentially records all changes made to the database in the Redo Log Buffer. The redo entries are written from the redo log buffer to the current online redo log file group by the LGWR process. LGWR writes under the following situations.

• When a transaction commits
• When the redo log buffers one-third full.
• When there is more than a megabyte of changed records in the Redo Log Buffer.
• Before the DBWn writes modified blocks in the database buffer cache to the Data Files.

Online redo log files are used in a cyclic fashion. Each Online redo log file group is identified by a log sequence number that is overwritten each time the log is reused.

Log Switches : 

LGWR writes to the online redo log files sequentially. When the current online redo log file group is filled, LGWR begins writing to the next group. This is called a log switch. 

When the last available online redo log file is filled. LGWR returns to the first online redo log file group and starts writing again.

Checkpoints :

During the checkpoint below activity happens and can say characteristics of Checkpoint : 

• DBWn writes a number of dirty database buffers, which are covered by the log that is being checkpointed, to the data file.
• The checkpoint backup ground process CKPT updates the control file to reflect that is it has completed a checkpoint successfully. If the checkpoint is caused by a log switch, CKPT also updates the headers of the data file.

Checkpoints can occur for all data files in the database or only for the specific data file.

In the following situation, checkpoints occur, for Example :

• At every log switch
• When an instance has been shut down with the normal, transactional, or Immediate option.
• When forced by setting the FAST_START_MTTR_TRAGET initialization parameter.
• When manually requested by the database administrator
• When the ALTER TABLESPACE [OFFLINE NORMAL | READ ONLY | BEGIN BACKUP] command causes checkpoint on specific data files.

Information about each checkpoint is recorded in the alert_SID.log file if the LOG_CHECKPOINTS_TO_ALERT initialization parameter is set to TRUE. The default value of FALSE  for this parameter does not log checkpoint.

Forcing Log Switches and Checkpoints :

• Forcing a log Switch by using Command :

 SQL> ALTER SYSTEM SWITCH LOGFILE;

• Checkpoint Can be Forced by :

- Setting FAST_START_MTTR_TARGET parameter.

FAST_START_MTTR_TARGET = 600; 

• ALTER SYSTEM CHECKPOINT  command.

SQL> ALTER SYSTEM CHECKPOINT;i

Log Switches and checkpoints are automatically performed at certain points in the operation of the database as identified previously.  However, a DBA can force a log switch or a checkpoint to occur.

Forcing Checkpoint :

FAST_START_MTTR_TARGET   Parameter replaces the deprecated parameters:

• FAST_START_IO_TARGET
• LOG_CHECKPOINT_TIMEOUT

These deprecated parameters must not be uses if the parameter FAST_START_MTTR_TARGET is used.

In the above example, the FAST_START_MTTR_TARGET parameter has been set 600 seconds, so that instance recovery should not take more than 600 seconds. The Database will adjust the other parameters to this goal.

Thank you !!