Tài liệu Managing Undo Data docx

After completing this lesson, you should be able to do the following: • Describe the purpose of undo data • Implement Automatic Undo Management • Create and configure undo segments • Obt

Managing Undo Data

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

• Describe the purpose of undo data

• Implement Automatic Undo Management

• Create and configure undo segments

• Obtain undo segment information

Managing Undo Data

• There are two methods for managing undo data:

– Automatic Undo Management

– Manual Undo Management

• The term undo was known as rollback in previous versions.

Undo Segment

Update transaction

Old image

New image Undo segment

Table

Undo Segments: Purpose

Transaction rollback Transaction

recovery Undo segment Read consistency

Types of Undo Segments

• SYSTEM: Used for objects in the SYSTEM tablespace

• Non-SYSTEM: Used for objects in other tablespaces:

– Auto mode: Requires an UNDO tablespace

– Manual mode:

Private: Acquired by a single instance Public: Acquired by any instance

• Deferred: Used when tablespaces are taken offline immediate,

temporary, or for recovery

Automatic Undo Management: Concepts

• Undo data is managed using an UNDO tablespace.

• You allocate one UNDO tablespace per instance with enough space for the workload of the instance.

• The Oracle server automatically maintains undo data within the UNDO tablespace.

Automatic Undo Management: Configuration

• Configure two parameters in the initialization file:

Automatic Undo Management:

Automatic Undo Management:

UNDO Tablespace

Create the UNDO tablespace with the database by

adding a clause in the CREATE DATABASE command:

Or create it later by using the CREATE UNDO

CREATE DATABASE db01

.

UNDO TABLESPACE undo1

DATAFILE '/u01/oradata/undoldb01.dbf' SIZE 20M AUTOEXTEND ON

CREATE UNDO TABLESPACE undo1

DATAFILE '/u01/oradata/undo1db01.dbf'

Automatic Undo Management:

Altering an UNDO Tablespace

• The ALTER TABLESPACE command can make changes to UNDO

tablespaces.

• The following example adds another data file to the UNDO tablespace:

ALTER TABLESPACE undotbs

ADD DATAFILE '/u01/oradata/undotbs2.dbf'

SIZE 30M

AUTOEXTEND ON;

Automatic Undo Management:

Switching UNDO Tablespaces

• You can switch from using one UNDO tablespace to another.

• Only one UNDO tablespace can be in assigned to a database at a time.

• More than one UNDO tablespace may exist within an instance, but only one can be active.

• Use the ALTER SYSTEM command for dynamic switching between UNDO tablespaces.

ALTER SYSTEM SET UNDO_TABLESPACE=UNDOTBS2;

Automatic Undo Management:

Dropping an UNDO Tablespace

• The DROP TABLESPACE command drops an UNDO tablespace.

• An UNDO tablespace can only be dropped if it is currently not in use by any instance.

• To drop an active UNDO tablespace:

– Switch to a new UNDO tablespace.

– Drop the tablespace after all current transactions are

complete.

DROP TABLESPACE UNDOTBS2;

Automatic Undo Management:

Other Parameters

• UNDO_SUPPRESS_ERRORS parameter:

– Set to TRUE, this parameter suppresses errors while attempting to execute manual operations in AUTO mode.

• UNDO_RETENTION parameter:

– This parameter controls the amount of undo data to

retain for consistent read.

Undo Data Statistics

SELECT end_time,begin_time,undoblks

FROM v$undostat;

END_TIME BEGIN_TIME UNDO - - - 22-JAN-01 13:44:18 22-JAN-01 13:43:04 19 22-JAN-01 13:43:04 22-JAN-01 13:33:04 1474 22-JAN-01 13:33:04 22-JAN-01 13:23:04 1347 22-JAN-01 13:23:04 22-JAN-01 13:13:04 1628 22-JAN-01 13:13:04 22-JAN-01 13:03:04 2249 22-JAN-01 13:03:04 22-JAN-01 12:53:04 1698 22-JAN-01 12:53:04 22-JAN-01 12:43:04 1433 22-JAN-01 12:43:04 22-JAN-01 12:33:04 1532 22-JAN-01 12:33:04 22-JAN-01 12:23:04 1075

Automatic Undo Management:

Sizing an UNDO Tablespace

Determining a size for the UNDO tablespace requires three pieces of information:

• (UR) UNDO_RETENTION in seconds

• (UPS) Number of undo data blocks generated per second

• (DBS) Overhead varies based on extent and file size (db_block_size)

Automatic Undo Management:

• UNDO_POOL, a Resource Manager directive, defines the amount of space

allowed for a resource group.

• When a group exceeds its limit, no new transactions are possible for the group, until undo space is freed by current transactions which are either completing or aborting.

Obtaining Undo Segment Information

• Information about undo segments can be obtained by querying the following views:

Summary

In this lesson, you should have learned how to:

• Configure Automatic Undo Management

• Create an UNDO tablespace

• Properly size an UNDO tablespace

• Obtain undo segment information

Practice 10 Overview

This practice covers the following topics:

• Creating an UNDO tablespace

• Switching between UNDO tablespaces

• Dropping an UNDO tablepsace