DBAI Redo Files

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Maintaining Redo Log Files


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Objectives

After completing this lesson, you should be able todo the following:

Explain the purpose of online redo log files

Outline the structure of online redo log files

Control log switches and checkpoints

Multiplex and maintain online redo log files

Manage online redo logs files with OMF


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Using Redo Log Files

Redo log files record all changes made to data andprovide a recovery mechanism from a system ormedia failure.

Redo log files are organized into groups.

An Oracle database requiresat least two groups.

Each redo log within agroup is called a member. Redo

log files

Database


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Structure of Redo Log Files

Group 2 Group 3Group 1

Member

Disk 1

Disk 2Member

Member Member

MemberMember


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How Redo Logs Work

Redo logs are used in a cyclic fashion.

When a redo log file is full, LGWR will move to thenext log group.

This is called a log switch

Checkpoint operation also occurs

Information is written to the control file


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Forcing Log Switches and Checkpoints

Log switches can be forced using theALTERSYSTEM SWITCH LOGFILE command.

Checkpoints can be forced using:

Setting FAST_START_MTTR_TARGET parameter

ALTER SYSTEM CHECKPOINT command

ALTER SYSTEM CHECKPOINT;


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log2b.rdolog1b.rdo

Adding Online Redo Log Groups

Group 1 Group 2 Group 3

ALTER DATABASE ADD LOGFILE GROUP 3('$HOME/ORADATA/u01/log3a.rdo','$HOME/ORADATA/u02/log3b.rdo')

SIZE 1M;

log3b.rdo

log2a.rdo log3a.rdolog1a.rdo


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Adding Online Redo Log Members

ALTER DATABASE ADD LOGFILE MEMBER'$HOME/ORADATA/u04/log1c.rdo' TO GROUP 1,'$HOME/ORADATA/u04/log2c.rdo' TO GROUP 2,'$HOME/ORADATA/u04/log3c.rdo' TO GROUP 3;

log2c.rdo

log2a.rdo

Group 1

log1c.rdo

log1b.rdo

log1a.rdo

Group 2

log2c.rdo

log2b.rdo

log2a.rdo

Group 3

log3c.rdo

log3b.rdo

log3a.rdo


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log1a.rdo log1a.rdo

Dropping Online Redo

Log Members

ALTER DATABASE DROP LOGFILE MEMBER

'$HOME/ORADATA/u04/log

3c.rdo

';

log2c.rdolog1c.rdo

Group 1 Group 2

log1b.rdo log1b.rdo


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Clearing, Relocating, or Renaming

Online Redo Log Files

Clearing online redo log files:

Relocating or renaming online redo log files canbe accomplished by adding new members anddropping old members.

ALTER DATABASE CLEAR LOGFILE'$HOME/ORADATA/u01/log2a.rdo';


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Online Redo Log Configuration

Group 2 Group 3Group 1

Member

Member

Member Member

Member

Disk 3Disk 2Disk 1

??


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Managing Online Redo Logs with OMF

A complete group can be added with no filespecification:

If a group is dropped, all the corresponding OMFfiles are deleted at the OS level:

ALTER DATABASE ADD LOGFILE;

ALTER DATABASE DROP LOGFILE GROUP 3;


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Archived Redo Log Files

Filled online redo log files can be archived.

Two advantages exist to archiving redo logs:

Recovery: A database backup, together with online

and archived redo log files can guarantee recoveryof all committed transactions.

Backup: Can be performed while the database isopen.

By default a database is created inNOARCHIVELOG

mode.


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Archived Redo Log Files

Archiving redo log files is accomplished byARCn(Archiver) or manually through SQL statements.

An entry in the control file recording the archive

log name, log sequence number, and high and lowSCN number is made whenever a redo log issuccessfully archived.

A filled redo log file cannot be reused until acheckpoint has taken place and the redo log file

has been backed up the ARCn process. Archived redo log files can be multiplexed.

Archived redo log files must be maintained by theDBA.


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Summary

In this lesson, you should have learned how to:

Explain the use of online redo log files

Obtain redo log information

Control log switches and checkpoints

Multiplex and maintain online redo log files

Manage online redo log files with OMF


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Practice 7 Overview

This practice covers the following topics:

Creating and adding redo log file groups andmembers.

Dropping redo log file groups and members.


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Documents

DBAI Redo Files