Redo Log Files

 What Oracle says on Redo Log File???

The most crucial and vital structure for recovery operations is the online redo log, which consists of two or more pre-allocated files that store all changes made to the database as they occur. Every instance of an Oracle database has an associated online redo log to protect the database in case of an instance failure.

What is a redo log thread?

Each database instance has its own online redo log groups. These online redo log groups, multiplexed or not, are called an instance's thread of online redo. In typical configurations, only one database instance accesses an Oracle database, so only one thread is present. When running Oracle Real Application Clusters, however, two or more instances concurrently access a single database and each instance has its own thread. The relation ship between Oracle Instance and Database is many-to-one. More than one Instance can access a Database. This kind of configuration is called Parallel Server Configuration.

What those files contain??

Online redo log files are filled with redo records. A redo record, also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database. For example, if you change a salary value in an employee table, you generate a redo record containing change vectors that describe changes to the data segment block for the table, the rollback segment data block, and the transaction table of the rollback segments.

Redo entries record data that you can use to reconstruct all changes made to the database, including the rollback segments. Therefore, the online redo log also protects rollback data.

When you recover the database using redo data, Oracle reads the change vectors in the redo records and applies the changes to the relevant blocks.

Redo records are buffered in a circular fashion in the redo log buffer of the SGA and are written to one of the online redo log files by the Oracle background process Log Writer (LGWR). Whenever a transaction is committed, LGWR writes the transaction's redo records from the redo log buffer of the SGA to an online redo log file, and a system change number (SCN) is assigned to identify the redo records for each committed transaction. Only when all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed.

Redo records can also be written to an online redo log file before the corresponding transaction is committed. If the redo log buffer fills, or another transaction commits, LGWR flushes all of the redo log entries in the redo log buffer to an online redo log file, even though some redo records may not be committed. If necessary, Oracle can roll back these changes.

How Oracle Writes to the Online Redo Log?

The online redo log of a database consists of two or more online redo log files. Oracle requires
a minimum of two files to guarantee that one is always available for writing while the other is being archived (if in ARCHIVELOG mode).

LGWR writes to online redo log files in a circular fashion. When the current online redo log file fills, LGWR begins writing to the next available online redo log file. When the last available online redo log file is filled, LGWR returns to the first online redo log file and writes to it, starting the cycle again. Filled online redo log files are available to LGWR for reuse depending on whether archiving is enabled. There can be contention between filling up of the on line redo log files and archiving of the redo log files, if they filled faster than they are written to the archived log file. This because online log file written in Oracle blocks and archives are written in OS blocks

If archiving is disabled (NOARCHIVELOG mode), a filled online redo log file is available once the changes recorded in it have been written to the data files.

If archiving is enabled (ARCHIVELOG mode), a filled online redo log file is available to LGWR once the changes recorded in it have been written to the datafiles and once the file has been archived.

What is meant by Active (Current) and Inactive Online Redo Log Files???

At any given time, Oracle uses only one of the online redo log files to store redo records written from the redo log buffer. The online redo log file that LGWR is actively writing to is called the current online redo log file.

Online redo log files that are required for instance recovery are called active online redo log files. Online redo log files that are not required for instance recovery are called inactive.

If you have enabled archiving (ARCHIVELOG mode), Oracle cannot reuse or overwrite an active online log file until ARCn has archived its contents. If archiving is disabled (NOARCHIVELOG mode), when the last online redo log file fills writing continues by overwriting the first available active file.

Which parameter influences the log switches??

LOG_CHECKPOINT_TIMEOUT specifies the amount of time, in seconds, that has passed since the incremental checkpoint at the position where the last write to the redo log (sometimes called the tail of the log) occurred. This parameter also signifies that no buffer will remain dirty (in the cache) for more than integer seconds. This is time based switching of the log files.

LOG_CHECKPOINT_INTERVAL specifies the frequency of checkpoints in terms of the number of redo log file blocks that can exist between an incremental checkpoint and the last block written to the redo log. This number refers to physical operating system blocks, not database blocks. This block based switching of the log files.

How do I respond the redo log failures???

How to add a redo log file group and member??

Suppose you are to add group 5 with 2 members the command is:

ALTER DATABASE
ADD LOGFILE GROUP 10
('c:oracleoradatawhsredoredo_05_01.log',
'd:oracleoradatawhsredoredo_05_02.log')
SIZE 100M;

This command is used to add another member to the group already existing.

ALTER DATABASE ADD LOGFILE MEMBER
'c:oracleoradatawhsredoredo_05_03.log'
TO GROUP 5;

How to move a redo log file from one destination to another destination??

01. Shutdown database normal/immediate but not abort.
Shutdown immediate;
02. Copy the online redo log files to the new location.
Unix use mv command
Windows move command
03. Startup MOUNT database logging in as sysdba (do not open the database)
startup mount pfile=<initialization parameter file with path>

04. Issue the following statement
Ex
You are changing the file from c:oracleoradataredologs to c:oracleoradatawhsredologs and like wise on d: drive.

ALTER DATABASE
RENAME FILE
'c:oracleoradataredologsredo_01_01.log',
'd:oracleoradataredologsredo_01_02.log'
TO
'c:oracleoradatawhsredologsredo_01_01.log',
'd:oracleoradatawhsredologsredo_01_02.log'
/

05. Open the database

alter database open;

How to drop a redo log file group or/and member???

To drop an online redo log group, you must have the ALTER DATABASE system privilege. Before dropping an online redo log group, consider the following restrictions and precautions:

An instance requires at least two groups of online redo log files, regardless of the number of members in the groups. (A group is one or more members.)

You can drop an online redo log group only if it is inactive. If you need to drop the current group, first force a log switch to occur.

Make sure an online redo log group is archived (if archiving is enabled) before dropping it. To see whether this has happened, use the V$LOG view.

SELECT GROUP#, ARCHIVED, STATUS FROM V$LOG;

GROUP# ARC STATUS
--------- --- ----------------
1 YES ACTIVE
2 NO CURRENT
3 YES INACTIVE
4 YES INACTIVE

Drop an online redo log group with the SQL statement ALTER DATABASE with the DROP LOGFILE clause.

The following statement drops redo log group number 3:

ALTER DATABASE DROP LOGFILE GROUP 3;

When an online redo log group is dropped from the database, and you are not using the Oracle Managed Files feature, the operating system files are not deleted from disk. The control files of the associated database are updated to drop the members of the group from the database structure. After dropping an online redo log group, make sure that the drop completed successfully, and then use the appropriate operating system command to delete the dropped online redo log files.
To drop an online redo log member, you must have the ALTER DATABASE system privilege. Consider the following restrictions and precautions before dropping individual online redo log members:

It is permissible to drop online redo log files so that a multiplexed online redo log becomes temporarily asymmetric. For example, if you use duplexed groups of online redo log files, you can drop one member of one group, even though all other groups have two members each. However, you should rectify this situation immediately so that all groups have at least two members, and thereby eliminate the single point of failure possible for the online redo log.
An instance always requires at least two valid groups of online redo log files, regardless of the number of members in the groups. (A group is one or more members.) If the member you want to drop is the last valid member of the group, you cannot drop the member until the other members become valid. To see a redo log file's status, use the V$LOGFILE view. A redo log file becomes INVALID if Oracle cannot access it. It becomes STALE if Oracle suspects that it is not complete or correct. A stale log file becomes valid again the next time its group is made the active group. You can drop an online redo log member only if it is not part of an active or current group. If you want to drop a member of an active group, first force a log switch to occur. Make sure the group to which an online redo log member belongs is archived (if archiving is enabled) before dropping the member. To see whether this has happened, use the V$LOG view.

To drop specific inactive online redo log members, use the ALTER DATABASE statement with the DROP LOGFILE MEMBER clause.

The following statement drops the redo log 'redo_01_01.log' for group 01 member 01

ALTER DATABASE DROP LOGFILE MEMBER 'c:oracleoradataredologsredo_01_01.log'

When an online redo log member is dropped from the database, the operating system file is not deleted from disk. Rather, the control files of the associated database are updated to drop the member from the database structure. After dropping an online redo log file, make sure that the drop completed successfully, and then use the appropriate operating system command to delete the dropped online redo log file.

To drop a member of an active group, you must first force a log switch and as a result that member becomes inactive.

How can I force the log switch??
ALTER SYSTEM SWITCH LOGFILE;

How can I Clear an Online Redo Log File??

ALTER DATABASE CLEAR LOGFILE GROUP 3;

This statement overcomes two situations where dropping redo logs is not possible:
(1) If there are only two log groups
(2) The corrupt redo log file belongs to the current group
(3) If the corrupt redo log file has not been archived, use the UNARCHIVED keyword in the statement.

ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 3;

Which Metadata Views I am to refer for redo log files????