10 Managing Rollback Segments. 10-2 Objectives Planning the number and size of rollback segments...

1010

Managing Rollback Segments Managing Rollback Segments

10-10-22

ObjectivesObjectives

• Planning the number and size of rollback segments

• Creating rollback segments using appropriate storage settings

• Maintaining rollback segments• Obtaining rollback segment information

from the data dictionary• Troubleshooting rollback segment

problems

• Planning the number and size of rollback segments

• Creating rollback segments using appropriate storage settings

• Maintaining rollback segments• Obtaining rollback segment information

from the data dictionary• Troubleshooting rollback segment

problems

10-10-33

Rollback Segment (Introduction)

Rollback Segment (Introduction)

•Used for delete and update operations only

•Used to ‘undo’ a transaction

•Used to save the old value when a process

changes data.

•Stores location and value of data before

modification

•A transaction can use only one rollback segment

10-10-44

Rollback Segment Rollback Segment

Update Update transactiontransaction

Old Old imageimage

New New imageimage

Rollback Rollback segmentsegment

TableTable

10-10-55

Rollback Segment (Steps) Rollback Segment (Steps)

•Data block brought into DB buffer

•Rollback segment brought into DB buffer

•Old data written to the rollback segment

•Update made to data block

10-10-66

Rollback Segments: PurposeRollback Segments: Purpose

Transaction Transaction rollbackrollback

Transaction Transaction recovery recovery

(when (when instance instance

fails)fails)

Rollback Rollback segmentsegment

Read Read consistencyconsistency

10-10-77

Read-ConsistencyRead-Consistency

Image at statement Image at statement commencementcommencement

New imageNew image

TableTableSELECT * SELECT * FROM tableFROM table

10-10-88


•When Oracle begins execution of a SELECT statement, it determines the current SCN

•Ensures that changes not committed before the SCN are not processed by the statement

•In the case of a long running transaction, Oracle constructs a read-consistent image of the block by retrieving the before-image and applying changes to it in memory

10-10-99


•Read consistency is always provided for a SQL statement

•Request read consistency for a transaction:

SET TRANSACTION READ ONLY;

Or

SET TRANSACTION SERIALIZABLE;

10-10-1010

Types of Rollback SegmentsTypes of Rollback Segments

• SYSTEM: used for objects in the SYSTEM tablespace; created in the SYSTEM ts

• Non-SYSTEM: objects in other tablespaces

– At least one needed

– Private: acquire by a single instance

– Public: acquired by any instance (used with parallel server)

• SYSTEM: used for objects in the SYSTEM tablespace; created in the SYSTEM ts

• Non-SYSTEM: objects in other tablespaces

– At least one needed

– Private: acquire by a single instance

– Public: acquired by any instance (used with parallel server)

10-10-1111

Transactions and Rollback Segments

Transactions and Rollback Segments

• Transactions need rollback segments to be assigned:– SET TRANSACTION Use Rollback Segment rbs1

• If no request is made, Oracle uses segment with fewest transactions

• Transactions use extents in a circular fashion

• More than one transaction can write to the same extent of a segment; however each block contains information from only one transaction

• Transactions need rollback segments to be assigned:– SET TRANSACTION Use Rollback Segment rbs1

• If no request is made, Oracle uses segment with fewest transactions

• Transactions use extents in a circular fashion

• More than one transaction can write to the same extent of a segment; however each block contains information from only one transaction

10-10-1212

Transactions and Rollback Segments Example

Transactions and Rollback Segments Example

Transaction 1Transaction 1

Active extentActive extent Inactive extentInactive extent

4 3

1 2

Transaction 2Transaction 2

Begin at extent 3

When full, move to extent 4 (called a wrap)

When that fills, can use the 1st extent if it is free or inactive

10-10-1313

Growth of Rollback SegmentsGrowth of Rollback Segments

Active extentActive extent

Inactive extentInactive extent

1 2

4 3

1 2

3

4

5

New extentNew extent

Cannot skip an extent; if next extent is being used, an additional extent is allocated . This is called an extend. Can grow this way until it reaches the maximum number of extents specified in the MAXEXTENTS parameter

10-10-1414

Shrinkage of Rollback SegmentsShrinkage of Rollback Segments

Active extentActive extent

Inactive extentInactive extent

1 2

6 3

1

2

34

5

6

OptimalOptimal

10-10-1515

Shrinkage of Rollback SegmentsShrinkage of Rollback Segments

•Deallocation of extents is not done as soon as transactions end.

•Deallocation performed when head moves from one extent to next and:

•Current size of rollback segment > OPTIMAL

•There are contiguous inactive extents

10-10-1616

Creating Rollback SegmentsCreating Rollback Segments

CREATE ROLLBACK SEGMENT rbs01

TABLESPACE rbs

STORAGE (

INITIAL 100K NEXT 100K OPTIMAL 4M

MINEXTENTS 20 MAXEXTENTS 100);

CREATE ROLLBACK SEGMENT rbs01

TABLESPACE rbs

STORAGE (

INITIAL 100K NEXT 100K OPTIMAL 4M

MINEXTENTS 20 MAXEXTENTS 100);

Example

MINEXTENTS: at least 2; PCTINCREASE not specified: always 0

Always use INITIAL = NEXT;

OPTIMAL specifies size in bytes that the rollback segment must shrink to, if possible

AVOID setting MAXEXTENTS to UNLIMITED: can cause unnecessary extension due to program error

10-10-1717

Planning Rollback Segments: Number

Planning Rollback Segments: Number

• OLTP

– Small, but many segments

– One RBS/4 transactions

• Batch

– Large, but few

• Always place rollback segments in a separate exclusive tablespace

• Always use INITIAL=NEXT

• PCTINCREASE cannot be specified (always 0)

• Set an OPTIMAL value

• OLTP

– Small, but many segments

– One RBS/4 transactions

• Batch

– Large, but few

• Always place rollback segments in a separate exclusive tablespace

• Always use INITIAL=NEXT

• PCTINCREASE cannot be specified (always 0)

• Set an OPTIMAL value

10-10-1818

Planning Rollback Segments: Number of Extents

Planning Rollback Segments: Number of Extents

0.000.000.000.00

0.100.100.100.10

0.200.200.200.20

0.300.300.300.30

0.400.400.400.40

0.500.500.500.50

0000 10101010 20202020 30303030 40404040

Number of extentsNumber of extentsNumber of extentsNumber of extents

ProbabilityProbabilityof extendingof extending

10-10-1919

Bringing Rollback Segments Online

Bringing Rollback Segments Online

• Use the following command to make a rollback segment available (when created, they are offline):

• Specify the following initialization parameter to ensure rollback segments are brought online at STARTUP:

• Use the following command to make a rollback segment available (when created, they are offline):

• Specify the following initialization parameter to ensure rollback segments are brought online at STARTUP:

ALTER ROLLBACK SEGMENT rbs01 ONLINE;ALTER ROLLBACK SEGMENT rbs01 ONLINE;

ROLLBACK_SEGMENTS=(rbs01, rbs02)ROLLBACK_SEGMENTS=(rbs01, rbs02)

10-10-2020

How Instances Acquire Rollback Segments when DB is opened

How Instances Acquire Rollback Segments when DB is opened

Bring allacquired

RBSs ONLINE

Acquire namedprivate

RBS

Are there enoughRBS ?

AcquirepublicRBS

Computerequirednumberof RBS

Yes

No

10-10-2121

Changing Rollback Segment Storage Settings

Changing Rollback Segment Storage Settings

ALTER ROLLBACK SEGMENT rbs01

STORAGE( MAXEXTENTS 200 );


STORAGE( MAXEXTENTS 200 );

Use ALTER ROLLBACK SEGMENTUse ALTER ROLLBACK SEGMENT

10-10-2222

Manually Deallocating Space from Rollback Segments

Manually Deallocating Space from Rollback Segments


SHRINK TO 4M;


SHRINK TO 4M;

If OPTIMAL is specified, Oracle will attempt to deallocate

Use ALTER ROLLBACK SEGMENT

(if extents are active, may not shrink to the requested size)

If OPTIMAL is specified, Oracle will attempt to deallocate

Use ALTER ROLLBACK SEGMENT

(if extents are active, may not shrink to the requested size)

10-10-2323

Taking Rollback Segment Offline

Taking Rollback Segment Offline


OFFLINE;


OFFLINE;

Take a rollback segment offline to makeit unavailable.Take a rollback segment offline to makeit unavailable.

10-10-2424

Dropping Rollback SegmentsDropping Rollback Segments

A rollback segment must be offline before it can be dropped.A rollback segment must be offline before it can be dropped.

DROP ROLLBACK SEGMENT rbs01;DROP ROLLBACK SEGMENT rbs01;

10-10-2525

Rollback Segments in the Database


DBA_ROLLBACK_SEGS

• Identification

– SEGMENT_ID

– SEGMENT_NAME

• Location, type, and status

– TABLESPACE_NAME

– OWNER (PUBLIC or SYS)

– STATUS (ONLINE or OFFLINE)

DBA_ROLLBACK_SEGS

• Identification

– SEGMENT_ID

– SEGMENT_NAME

• Location, type, and status

– TABLESPACE_NAME

– OWNER (PUBLIC or SYS)

– STATUS (ONLINE or OFFLINE)

10-10-2626



Select segment_name, tablespace_name,

owner, status

From dba_rollback_segs;

Select segment_name, tablespace_name,

owner, status

From dba_rollback_segs;

10-10-2727

Rollback Segment StatisticsRollback Segment Statistics

V$ROLLNAMEUSNNAME

V$ROLLNAMEUSNNAME

V$ROLLSTAT USN

EXTENTSRSSIZE XACTS OPTSIZE HWMSIZE AVEACTIVESTATUS CUREXT CURBLK

V$ROLLSTAT USN

EXTENTSRSSIZE XACTS OPTSIZE HWMSIZE AVEACTIVESTATUS CUREXT CURBLK

10-10-2828

Rollback Segment: Current Activity Rollback Segment: Current Activity

V$SESSIONSADDRUSERNAMESIDSERIAL#

V$SESSIONSADDRUSERNAMESIDSERIAL#

V$TRANSACTION SES_ADDRXIDUSN UBAFIL UBABLK UBASQN UBAREC STATUS USED_UBLK USED_UREC

V$TRANSACTION SES_ADDRXIDUSN UBAFIL UBABLK UBASQN UBAREC STATUS USED_UBLK USED_UREC

10-10-2929

Rollback Segment ProblemsRollback Segment Problems

• Insufficient space for transactions

• Read-consistency error

• Blocking transaction

• Error in taking tablespace offline

• Insufficient space for transactions

• Read-consistency error

• Blocking transaction

• Error in taking tablespace offline

10-10-3030

Insufficient Space for TransactionsInsufficient Space for Transactions

• No space in tablespace to extend– Extend data files– Allow automatic extension of data

files– Add data files

• MAXEXTENTS reached for segment– Increase MAXEXTENTS– Recreate segments with larger extent

sizes

• No space in tablespace to extend– Extend data files– Allow automatic extension of data

files– Add data files

• MAXEXTENTS reached for segment– Increase MAXEXTENTS– Recreate segments with larger extent

sizes

10-10-3131

Read-Consistency Error(snapshot too old)

Read-Consistency Error(snapshot too old)

Image at statement Image at statement commencementcommencement

New imageNew image

TableTableSELECT * SELECT * FROM tableFROM table

Reused Reused blockblock

A long running Trans

Another transaction commits

Rollback segment is reused

Soln: Higher MINEXTENTS; larger extents; higher OPTIMAL value

10-10-3232

Blocking Session Blocking Session

Extent 3

Existing extentExisting extent

New extentNew extent

4 1

3 2

1 2

3

4

5

Blocking session

A long running trans keeps an extent occupied and blocks other trans from writing to it; segment keeps extending

Soln: DBA intervention

10-10-3333

Error in Taking a Tablespace Offline Error in Taking a

Tablespace Offline

Cannot take tablespace containing active RBS offline

• Check rollback segments in the tablespace

• Find active transactions using these rollback segments

• Find session ID and serial number

• Terminate the session, if necessary

Cannot take tablespace containing active RBS offline

• Check rollback segments in the tablespace

• Find active transactions using these rollback segments

• Find session ID and serial number

• Terminate the session, if necessary

10-10-3434

Summary Summary

• Creating adequate rollback segments

• Troubleshooting rollback segment problems

• Creating adequate rollback segments

• Troubleshooting rollback segment problems

10 Managing Rollback Segments. 10-2 Objectives Planning the number and size of rollback segments...

Documents

Transcript of 10 Managing Rollback Segments. 10-2 Objectives Planning the number and size of rollback segments...