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Chapter 5The Redo Log Files

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Objectives

Learn to describe redo log files, groups, and members

Manage redo log groups and members

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Introduction to Online Redo Log Files

Online redo log files store details as the database is used including:

Checkpoints Data Changes (DML) Structural changes (DDL) Data file changes

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Oracle9i Architecture: A Typical Server

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Introduction to Online Redo Log Files

Components:•Redo log group•Online redo log file or member•Archived redo log file•ARCn

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Introduction to Online Redo Log Files

Components:•Redo log buffer•LGWR•CKPT

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Introduction to Online Redo Log Files

File 1 is written to File A by the ARCn process after the log switch is complete

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Introduction to Online Redo Log Files

File 1 and File 3 are written to by the LGWR process simultaneouslyuntil both are full

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The Purpose of Redo Log Files

Redo log files aid in recovery from short term data loss such as brief power outage:

Store changes that may not be written to the data files yet

Store checkpoints to allow recovery to resynchronize data files with correct changes

Cannot completely handle recovery from major loss such as an entire data file

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The Purpose of Redo Log Files

Recovery procedures (overview):

Prior to failure: Perform full database backup regularly Run database in ARCHIVELOG mode

After failure: Restore from full backup Apply archived redo log files (oldest to newest) Apply online redo log file

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The Structure of Redo Log Files

Redo log buffer is flushed to the redo log file when: A transaction COMMITS The redo log buffer becomes 1/3 full The redo log buffer contains >1 M of

updated records A checkpoint occurs

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The Structure of Redo Log Files

Redo log file components: Redo record (also called redo entry)

Relates to one data block Made up of one or more change vectors

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Managing Redo Log Files

The COMMIT command tells the database to log the changes to the redo log file

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Log Switches and Checkpoints To manually signal a log switch:

ALTER SYSTEM SWITCH LOGFILE;

A log switch triggers a checkpoint Checkpoint is a signal to write all dirty

buffers to the appropriate files Checkpoint increments the System

Change Number (SCN)

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Log Switches and Checkpoints

How the SCN is used: SCN is recorded in the redo log buffer SCN is recorded in the header of each

data file that was written to During recovery, SCN of each datafile is

compared to the SCN in the redo log file If a match, the data file is up to date If redo log file has higher SCN, changes are

reapplied to the data file from the redo log file

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Multiplexing and Other Maintenance Multiplexed redo log files requires multiple

members in each group A log group continues to function as long as

it has at least one good member If all members of the current group become

damaged, the database forces a log switch The database shuts down immediately if a

log switch fails

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Adding a Member to a Group Add members to groups while the database is

running Do not specify SIZE because new members

are automatically assigned the same size as other members in the group

Command syntax:ALTER DATABASE ADD LOGFILE MEMBER '<X:\xxx>\<filename>'

TO GROUP <n>;

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Adding a New Group Add a new group while the database is

running Specify SIZE and group number List all members to be created with the

group (must be at least one) Command syntax:

ALTER DATABASE ADD LOGFILE GROUP <n>

( '<X:\xxx>\<filename>', '<X:\xxx>\<filename>')

SIZE <n>;

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Renaming or Moving a Redo Log File Must be done while the group is not

ACTIVE Steps:

1. Shut down the database:SHUTDOWN IMMEDIATE

2. Rename or relocate files using the operating system

3. Mount the databaseSTARTUP MOUNT

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Renaming or Moving a Redo Log File Steps (continued):

4. Inform database of changed file name:ALTER DATABASE

RENAME FILE '<X:\xxx>\<oldfilename1>',

'<X:\xxx>\<oldfilename2>'

TO '<X:\xxx>\<newfilename1>',

'<X:\xxx>\<newfilename2>';

5. Open the database:ALTER DATABASE OPEN;

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Dropping Redo Log Members or Groups

Reasons for dropping members or groups:

Bad disk, so must recreate member Tuning recommendation calls for

reducing members or groups Corrupted members can be deleted

and replaced later

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Dropping Redo Log Members or Groups Rules when dropping redo log members:

Each group must have at least one member The group affected must not be ACTIVE and (if in

ARCHIVELOG mode) must be archived

Rule when dropping groups: Database requires at least two groups The group affected must not be ACTIVE and (if in

ARCHIVELOG mode) must be archived

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Dropping Redo Log Members or Groups

Steps to drop redo log member:

1. Query V$LOG to confirm that group is inactive

2. Drop redo log member:ALTER DATABASE DROP LOGFILE MEMBER

'<X:\xxx>\<filename>';

3. Delete associated file in the operating system

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Dropping Redo Log Members or Groups

Steps to drop redo log group:

1. Query V$LOG to confirm that group is inactive: SELECT * FROM V$LOG;

2. Drop redo log group:ALTER DATABASE DROP LOGFILE GROUP <n>;

3. Delete associated file(s) in the operating system

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Dropping Redo Log Members or Groups

Additional notes: If group is ACTIVE, force log switch:

ALTER SYSTEM SWITCH LOGFILE;

If group still active, force checkpoint:ALTER SYSTEM CHECKPOINT;

An alternative to dropping a corrupt group and recreating the members:ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <n>;

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Archiving a Redo Log Group

Advantages of archiving redo log groups: Point-in-time recovery to a point earlier

than online redo log files contain The ability to query archived log files with

LogMiner The ability to set up and maintain a

standby database

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Finding Redo Log Information in Data Dictionary Views

States of a redo log group: UNUSED CURRENT ACTIVE CLEARING CLEARING_CURRENT INACTIVE

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Viewing Control File Data

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Chapter Summary Redo log files primarily contain information on

database changes

A database has at least two redo log groups

A redo log group has at least one member

Redo log files support automatic recovery from minor failures

A redo log group with multiple files is multiplexed

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Chapter Summary The SCN is incremented at a checkpoint Maintenance you can do with redo log files and

groups: Add a new file (member) to a group Add a new group Rename or relocate a member Drop a member Drop a group Clear a group

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Chapter Summary Placing a database in ARCHIVELOG

mode causes redo log files to be archived after a log switch

Many initialization parameters set the behavior of the archiving function

The V$LOG and V$LOGFILES dynamic performance views display information about redo log groups and members