Logfiles in Oracle

Redo Log files in oracle

•  A redo log is a file that is part of an Oracle database.
•  When a transaction is committed, the transaction's details in the redo log buffer is written to a redo log file. 
•    ---It is another name is Online-redologfiles.
•  As the Redo Log Buffer fills, the contents are written to Redo Log Files. 
•  This includes uncommitted transactions, undo segment data, and schema/object management information. 
•  During database recovery, information in Redo Log Files enable data that has not yet been written to datafiles to be recovered.  

  

A Redo Log File stores Redo Records (also called redo log entries).

• Each record consists of "Change vectors" that store information about:
• changes made to a database block. 
• undo block data.
• transaction table of undo segments.

Group status:-

• Unused – the Redo Log Group has never been used – this status only occurs for a newly added Redo Log Group.
• Current – The active Redo Log Group.it is currently writing lgwr to logfile.
• Active – the Redo Log Group is active, but not the current Group –                                                            it is needed for crash recovery and may be in use for block recovery.                                            It may not yet be archived.
• Clearing – the Log is being recreated after an ALTER DATABASE CLEAR LOGFILE command.
• Clearing_Current – the current Redo Log Group is being cleared of a closed group.
• Inactive – The Group is not needed for Instance Recovery.

Member Status:-

• Invalid – the file cannot be accessed and needs to be dropped and recreated.
• Stale – the contents of the file are incomplete – drop it and recreate it.
• Deleted – the file is no longer in use – you can use operating system commands to delete the associated operating system file.
• Blank – the file is in use.

Managing size of Redologfile:-
Minimum size for an On-line Redo Log File is 4MB.
Maximum size and Default size depends on the operating system.
If you set logfile size in 4mb,the switch occur more than 10 mintues.
---you need to set target of switching logfile.
   ALTER SYSTEM SET ARCHIVE_LAG_TARGET = 1200;Milli-seconds.

 
The database requires a minimum of two Logfile Group to guarantee.(if the database is in ARCHIVELOG mode).
And each group have minimum two member's,group have differnt information but member have same information with in a group



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

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

Adding Logfile Groups:-

alter database add logfile group 3 '/home/oracle/icici/log/log03a.log' size 10m;

Adding a member to alredy exits groups:-
alter database add logfile member '/home/oracle/icici/log/log03b.log' to group 3;

alter database add logfile member '/home/oracle/icici/log/log03b.log' to group 3,'/home/oracle/icici/log/log03c.log' to group 4;

Dropping Redo Log File Groups and Files:-

ALTER DATABASE DROP LOGFILE GROUP 3;

Drop LOGFILE member from Group
ALTER DATABASE DROP LOGFILE MEMBER' /u01/student/dbockstd/oradata/USER350redo01a.log';


Managing Redo log group's

SELECT group#, status FROM v$log;
select * from v$logfile;