Manually Managing Undo Data (Rollback Segments) pot

After completing this lesson, you should be able to do the following: • Create rollback segments using appropriate storage settings • Maintain rollback segments • Plan the number and si

B

Manually Managing Undo Data

(Rollback Segments)

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

• Create rollback segments using appropriate storage

settings

• Maintain rollback segments

• Plan the number and size of rollback segments

• Troubleshoot common rollback segment problems

Creating Rollback Segments

CREATE ROLLBACK SEGMENT rbs01

TABLESPACE rbs

STORAGE (

INITIAL 100K

MINEXTENTS 20

MAXEXTENTS 100

OPTIMAL 2000K );

Transactions and Rollback Segments

Transaction 1

Active extent Inactive extent

Transaction 2

Growth of Rollback Segments

Active extent Inactive extent

3 4

5

New extent

Shrinkage of Rollback Segments

Active extent Inactive extent

1

2 3

4 5

6

OPTIMAL

Bringing Rollback Segments Online

• Use the following command to make a rollback

segment available:

–

• Specify the following initialization parameter to

ensure that rollback segments are brought online at startup:

ROLLBACK_SEGMENTS=(rbs01, rbs02)

ALTER ROLLBACK SEGMENT rbs01 ONLINE;

How Instances Acquire Rollback Segments

Bring all acquired rollback segments

online.

Acquire

named private

rollback

segments.

Are there enough RBS’s?

Acquire public rollback segments.

Compute the required number

of rollback segments.

Yes

No

Changing Rollback Segment

Storage Settings

• Use the ALTER ROLLBACK SEGMENT command.

• You can change OPTIMAL or MAXEXTENTS.

ALTER ROLLBACK SEGMENT rbs01

STORAGE( MAXEXTENTS 200 );

Deallocating Space From

Rollback Segments

• Use the ALTER ROLLBACK SEGMENT command.

• If extents are active, they might not shrink to the

requested size.

ALTER ROLLBACK SEGMENT rbs01

SHRINK TO 4M;

Taking Rollback Segment Offline

• Take a rollback segment offline to make it

unavailable.

• If transactions are using the rollback segment, the

status is temporarily changed to PENDING OFFLINE ALTER ROLLBACK SEGMENT rbs01

OFFLINE;

Dropping Rollback Segments

• A rollback segment must be offline before it can be

dropped.

DROP ROLLBACK SEGMENT rbs01;

Planning Rollback Segments: Number

• OLTP

– Many small rollback segments

– Four transactions per rollback segment

– Up to ten transactions per rollback segment

• Batch

– Few large rollback segments

– One per transaction

Planning Rollback Segments:

Number of Extents

0.00 0.10 0.20 0.30 0.40 0.50

Probability

of extending

Rollback Segment Problems

• Insufficient space for transactions

• Read-consistency errors

• Blocking sessions

• Errors in taking a tablespace offline

Insufficient Space for Transactions

• No space in tablespace:

– Extend data files

– Enables automatic extension of data files

– Add data files

• MAXEXTENTS reached for segment

– Increase MAXEXTENTS

– Re-create segments with larger extent sizes

New image

Table FROM table SELECT *

Read-Consistency Errors

Reused block

Blocking Sessions

Extent 3

Existing extent

3 4

5

Blocking session

Errors in Taking a Tablespace Offline

You cannot take a tablespace offline if it contains an active rollback segment.

1 Determine which rollback segments are in the tablespace.

2 Take all of these rollback segments offline.

3 Find active transactions using these rollback

segments.

4 Find the session ID and serial number.

5 Terminate the session, if necessary.

6 Take the tablespace offline.