Redo Log files records every change made by users. This guarantees that we will not lose any change, even if it was not written in Data Files due to instance failure.The most crucial structure for recovery operations is the redo log, which consists of two or more preallocated files that store all changes made to the database as they occur. Every instance of an Oracle Database has an associated redo log to protect the database in case of an instance failure.
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.Redo records are buffered in a circular fashion in the redo log buffer of the SGA and are written to one of the redo log files by the Log Writer (LGWR) database background process. When the current redo log file fills, LGWR begins writing to the next available redo log file. When the last available redo log file is filled, LGWR returns to the first redo log file and writes to it, starting the cycle again.
The redo log of a database consists of two or more redo log files. The database requires a minimum of two files to guarantee that one is always available for writing while the other is being archived (if the database is in ARCHIVELOG mode).
As the part of circular writting, a log switch is the point at which the database stops writing to one redo log file and begins writing to another. Normally, a log switch occurs when the current redo log file is completely filled and writing must continue to the next redo log file. Oracle Database assigns each redo log file a new log sequence number every time a log switch occurs and LGWR begins writing to it.
Multiplexing is the procedure to keep the multiple copy of the redo log files for the confirmation of recovery steps in case of failure. It is implemented by creating groups of redo log files. A group consists of a redo log file and its multiplexed copies. Each identical copy is said to be a member of the group. Each redo log group is defined by a number, such as group 1, group 2, and so on.
For the new allocation, Block Size is maintained for redo log files. It might be 4K and 512B.
However, with a block size of 4K, there is increased redo wastage. In fact, the amount of redo wastage in 4K blocks versus 512B blocks is significant. You can determine the amount of redo wastage by viewing the statistics stored in the V$SESSTAT and V$SYSSTAT views.
SQL> SELECT name, value FROM v$sysstat WHERE name = 'redo wastage';
Beginning with Oracle Database 11g Release 2, you can specify the block size of online redo log files with the BLOCKSIZE keyword in the CREATE DATABASE, ALTER DATABASE, and CREATE CONTROLFILE statements. The permissible block sizes are 512, 1024, and 4096.
Adding Redo Log File:
To create a new group of redo log files, use the SQL statement ALTER DATABASE with the ADD LOGFILE clause. The following statement adds a new group of redo logs to the database:
ALTER DATABASE
ADD LOGFILE ('/oracle/dbs/log1c.rdo', '/oracle/dbs/log2c.rdo') SIZE 100M;
You can also specify the number that identifies the group using the GROUP clause:
ALTER DATABASE
ADD LOGFILE GROUP 10 ('/oracle/dbs/log1c.rdo', '/oracle/dbs/log2c.rdo')
SIZE 100M BLOCKSIZE 512;
To create new redo log members for an existing group, use the SQL statement ALTER DATABASE with the ADD LOGFILE MEMBER clause. The following statement adds a new redo log member to redo log group number 2:
ALTER DATABASE ADD LOGFILE MEMBER '/oracle/dbs/log2b.rdo' TO GROUP 2;
Notice that filenames must be specified, but sizes need not be. The size of the new members is determined from the size of the existing members of the group.
When using the ALTER DATABASE statement, you can alternatively identify the target group by specifying all of the other members of the group in the TO clause, as shown in the following example:
ALTER DATABASE ADD LOGFILE MEMBER '/oracle/dbs/log2c.rdo'
TO ('/oracle/dbs/log2a.rdo', '/oracle/dbs/log2b.rdo');
Steps for Renaming Redo Log Members:
- Shut Down the database: SHUTDOWN
- Copy the redo log files to the new location.
- Startup the database, mount, but do not open it.
- Rename the redo log members.
- Use the ALTER DATABASE statement with the RENAME FILE clause to rename the database redo log files.
RENAME FILE '/diska/logs/log1a.rdo', '/diska/logs/log2a.rdo'
TO '/diskc/logs/log1c.rdo', '/diskc/logs/log2c.rdo';
- Open the database for normal operation.
- The redo log alterations take effect when the database is opened.
Dropping Redo log File;
To drop a redo log member, you must have the ALTER DATABASE system privilege.
ALTER DATABASE DROP LOGFILE MEMBER '/oracle/dbs/log3c.rdo';
When a redo log member is dropped from the database, the operating system file is not deleted from disk.
Forcing Log Switches: ALTER SYSTEM SWITCH LOGFILE;
Clearning Log File:
The following statement clears the log files in redo log group number 3:
ALTER DATABASE CLEAR LOGFILE 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;
Redo Log Data Dictionary Views:
The following views provide information on redo logs.
View
|
Description
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V$LOG
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Displays the redo log file information from the control file
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V$LOGFILE
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Identifies redo log groups and members and member status
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V$LOG_HISTORY
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Contains log history information
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