oracle 8 database administration volume 1 instruction guide phần 9 pps

Oracle8: Database Administration 10-27 ......Obtaining Rollback Segment Information The OWNER column specifies the type of a rollback segment: OEM segments in the database.. Oracle8: Dat

Oracle8: Database Administration 10-27

Obtaining Rollback Segment Information

The OWNER column specifies the type of a rollback segment:

OEM

segments in the database

the rollback segment

Join the V$ROLLSTAT and V$ROLLNAME views to obtain the statistics of

the rollback segments currently used by the instance:

Example

SVRMGR> SELECT n.name, s.extents, s.rssize, s.optsize,

2> s.hwmsize, s.xacts, s.status

3> FROM v$rollname n, v$rollstat s

4> WHERE n.usn = s.usn;

NAME EXTENTS RSSIZE OPTSIZE HWMSIZE XACTS STATUS - - - - - - -

10-20 Copyright  Oracle Corporation, 1998 All rights reserved.

Rollback Segment Statistics

V$ROLLNAME

USN

NAME

V$ROLLSTAT USN

EXTENTS RSSIZE XACTS OPTSIZE HWMSIZE AVEACTIVE STATUS CUREXT CURBLK

Oracle8: Database Administration 10-29

Obtaining Rollback Segment Information

The columns in V$ROLLSTAT include :

Note

from V$ROLLSTAT view

ONLINE in DBA_ROLLBACK_SEGS

USN Rollback (undo) segment number; join with

V$ROLLNAME.USN to get the name EXTENTS Number of extents in the rollback segment

RSSIZE Current size of the segment in bytes

XACTS Number of transactions using this segment

OPTSIZE OPTIMAL value for the rollback segment

HWMSIZE High water mark; the maximum size in bytes to

which the segment grew since startup AVEACTIVE Current size of active extents, averaged over time STATUS Status of the rollback segment

(ONLINE indicates that the rollback segment is available for use PENDING OFFLINE indicates

offline has been executed and there are someactive transactions using the segment Therollback segment will be taken offline as soon

as all transactions using the rollback segmenthave beencompleted.)

CUREXT Current location of the rollback segment head; the

extent and block number CURBLK

To check the use of rollback segment by currently active transactions, join the

V$TRANSACTION and V$SESSION views:

Example

SVRMGR> SELECT s.username, t.xidusn, t.ubafil,

2> t.ubablk, t.used_ublk

3> FROM v$session s, v$transaction t

4> WHERE s.saddr = t.ses_addr;

- - - -

10-21 Copyright  Oracle Corporation, 1998 All rights reserved.

Rollback Segment: Current Activity

XIDUSN UBAFIL UBABLK UBASQN UBAREC STATUS

USED_UBLK USED_UREC

Oracle8: Database Administration 10-31

Obtaining Rollback Segment Information

The relevant columns in V$TRANSACTION and their descriptions are

shown below:

SES_ADDR Address of the session; can be joined to

V$SESSION.SADDR XIDUSN Rollback (Undo) Segment Number used by the

transaction; used as part of the transaction ID UBAFIL The columns UBAFIL, UBABLK, UBASQN, and

UBAREC specify the current location in the rollback segment that the transaction is writing to UBABLK

UBASQN

UBAREC

USED_UBLK Number of blocks of undo generated by the

transaction START_UEXT Rollback segment extent from which the

transaction started writing START_UBAFIL Rollback segment file number from which the

transaction started writing START_UBABLK Rollback segment block number from which the

transaction started writing

Troubleshooting Rollback Segment Problems

10-22 Copyright  Oracle Corporation, 1998 All rights reserved.

Rollback Segment Problems

• Insufficient space for transactions

• Read-consistency error

• Blocking transaction

• Error in taking tablespace offline

Oracle8: Database Administration 10-33

Troubleshooting Rollback Segment Problems

Possible Causes

A transaction cannot use multiple rollback segments and may fail because

there is insufficient space in the rollback segment (ORA-01562) This could

be caused by one of the following:

MAXEXTENTS and additional extents cannot be allocated

(ORA-01628)

Solution

If the tablespace does not have free space, increase the space available to the

tablespace by:

If a rollback segment cannot allocate more extents because the limit imposed

by MAXEXTENTS has been reached:

recurrence of the problem

10-23 Copyright  Oracle Corporation, 1998 All rights reserved.

Insufficient Space for Transactions

• No space in tablespace

– Extend data files

– Allow automatic extension of data

files

– Add data files

• MAXEXTENTS reached for segment

– Increase MAXEXTENTS

– Recreate segments with larger extent

sizes

Possible Causes

As shown in the section on Read-Consistency, the Oracle server guarantees

that a statement will only process data committed at a given time Any

changes that are not committed when the statement begins or are made after

the statement begins execution, will not be seen by the statement If the

Oracle server cannot construct a read-consistent image of data, the user will

receive an ORA-01555 SNAPSHOT TOO OLD error This error can occur

when the transaction that made the change has already committed and:

another transaction

10-24 Copyright  Oracle Corporation, 1998 All rights reserved.

Read-Consistency Error

Image at statement commencement New image

Table

SELECT *

block

Oracle8: Database Administration 10-35

Troubleshooting Rollback Segment Problems

Solution

Read-consistency errors can be minimized by ensuring that rollback

segments are created with:

Note that these errors cannot be avoided by increasing MAXEXTENTS

Instructor Note

There are other possible causes of the ORA-01555 error, which also occurs

when a read-consistent image cannot be constructed because the transaction

slot in the data block is reused

Possible Causes

When an extent in a rollback segment is filled up, the Oracle server attempts

to reuse the next extent in the ring Even if this new extent contains one

active entry—that is, an entry that was written by a transaction that is still

active—it cannot be used In these cases, a rollback segment allocates an

additional extent The transaction cannot skip an extent in the ring and

continue writing to a subsequent extent A transaction that has made only a

few changes, but has been idle for a long time could cause rollback segments

to grow even though there are many free extents In such situations there is a

lot of space wasted and a database administrator may need to intervene to

avoid excessive rollback segment growth

10-25 Copyright  Oracle Corporation, 1998 All rights reserved.

Blocking Session

Extent 3

Existing extent New extent

3 4

5 Blocking

session

Oracle8: Database Administration 10-37

Troubleshooting Rollback Segment Problems

Detecting Blocking Sessions

Query V$ROLLSTAT, V$SESSION, and V$TRANSACTION views to find

any blocking transactions

Example

SVRMGR> SELECT s.sid, s.serial#, t.start_time, t.xidusn,

s.username

2> FROM v$session s, v$transaction t, v$rollstat r

3> WHERE s.saddr = t.ses_addr

4> AND t.xidusn = r.usn

5> AND ((r.curext = t.start_uext-1) OR

6> ((r.curext = r.extents-1) AND t.start_uext=0));

- - - -

1 row selected.

Check if the transaction can be ended or whether the transaction needs to be

killed, and take the necessary action

Problem Diagnosis and Resolution

If a tablespace contains one or more active rollback segments, it cannot be

taken offline, and the session attempting to execute the statement will

receive an ORA-01546 error In this situation, perform the following steps:

the tablespace

2 Take all rollback segments in the tablespace offline

using these rollback segments

5 Terminate the sessions if necessary

6 Take the tablespace offline

10-26 Copyright  Oracle Corporation, 1998 All rights reserved.

Error in Taking a Tablespace Offline

Cannot take tablespace containing active

• Find session ID and serial number

• Terminate the session, if necessary

Oracle8: Database Administration 10-39

Summary

Summary

10-27 Copyright  Oracle Corporation, 1998 All rights reserved.

Summary

• Creating adequate rollback segments

• Troubleshooting rollback segment

problems

Quick Reference

Initialization parameters ROLLBACK_SEGMENTS

TRANSACTIONS TRANSACTIONS_PER_ROLLBACK_SEGMENTS MAX_ROLLBACK_SEGMENTS

Dynamic performance views V$ROLLNAME

V$ROLLSTAT V$TRANSACTION V$SESSION Data dictionary views DBA_ROLLBACK_SEGS

Commands CREATE ROLLBACK SEGMENT

ALTER ROLLBACK SEGMENT ONLINE ALTER ROLLBACK SEGMENT STORAGE ALTER ROLLBACK SEGMENT SHRINK ALTER ROLLBACK SEGMENT OFFLINE DROP ROLLBACK SEGMENT

SET TRANSACTION USE ROLLBACK SEGMENT

11

Managing Temporary

Segments

Oracle8: Database Administration 11-3

• Allocating space for temporary

segments within a database

• Obtaining temporary segment

information for a database or instance

Temporary Segment Usage

Temporary segments are used when statements such as the following are

executed and the Oracle server cannot perform the sorting needed in

The amount of memory used by a process for sorting is determined by the

SORT_AREA_SIZE initialization parameter If the sort volume exceeds this

size, several sort runs are needed, and intermediate results are stored on disk

Temporary segments are created and used by the Oracle server in the

11-3 Copyright  Oracle Corporation, 1998 All rights reserved.

Temporary Segment

Data file Memory

SORT_AREA_SIZE

Oracle8: Database Administration 11-5

Overview

Note

increasing it might increase the memory requirement significantly

Users.”

Oracle8: Performance Tuning.

Instructor Note

When using MTS, memory is allocated for sorts in the SGA

Types of Temporary Segments

Temporary segments may be created either in a PERMANENT tablespace or

in a TEMPORARY tablespace A user may be assigned one of these types of

tablespaces for sorting

Temporary Tablespace

A TEMPORARY tablespace is used exclusively for temporary segments and

cannot have any other type of segment They can be created as

TEMPORARY by using the following command:

CREATE TABLESPACE tablespace_name TEMPORARY

DATAFILE filespec [autoextend_clause]

[ , filespec [autoextend_clause]]

A PERMANENT tablespace can be changed into a TEMPORARY

tablespace by using:

11-4 Copyright  Oracle Corporation, 1998 All rights reserved.

Types of Temporary Segments

Temporary tablespace

Permanent tablespace

Memory

Oracle8: Database Administration 11-7

Types of Temporary Segments

The Oracle server may create a temporary segment in a PERMANENT

tablespace if the following conditions are satisfied:

tablespace for sorting

If a PERMANENT tablespace is used for sorting, an instance may have

more than one temporary segment in the tablespace

A temporary segment is dropped by SMON on completion of a statement

and space is released for use by other objects If PERMANENT tablespaces

are used for sorting, the free space in the tablespace may be highly

fragmented So it is recommended that this tablespace be used exclusively

• Reclaimed by SMON when the

statement completes execution

If TEMPORARY tablespaces are used for temporary segments, an instance

only creates one sort segment in this tablespace Several transactions

requiring disk sorts can use the same segment An extent cannot be shared

by multiple transactions

Use of Sort Segments

A sort segment is created by the first statement that uses a TEMPORARY

tablespace for sorting, after startup A sort segment created in a

TEMPORARY tablespace is released only at shutdown This decreases the

frequent allocation and deallocation of extents by operations needing sorts

and helps improve performance There is no limit to the number of extents

for a sort segment created in a TEMPORARY tablespace

Sort Extent Pool

11-6 Copyright  Oracle Corporation, 1998 All rights reserved.

Temporary Segments

in a Temporary Tablespace

• Known as sort segments

• Only one segment per tablespace per

instance

• Created when the first disk sort occurs

in the instance after startup

• Reused by several transactions based

on information in the Sort Extent Pool

• Released on instance shutdown

Oracle8: Database Administration 11-9

Allocating Space for Temporary Segments

Allocating Space for Temporary Segments

Create and use TEMPORARY tablespaces for sorting to improve

concurrency and to decrease frequent allocation and deallocation of space

The sizes of the extents used by a temporary segment are governed by the

DEFAULT STORAGE clause specified for the tablespace

Specifying Default Storage

Use the following guidelines to specify DEFAULT STORAGE:

SORT_AREA_SIZE to a temporary segment, a good value for the extent

size is (n*s + b)

where: n is a positive integer

s is the value of SORT_AREA_SIZE initialization parameter

b is the value of DB_BLOCK_SIZE initialization parameter

Using this value optimizes temporary segment usage by allowing for

sufficient space for a header block and multiple sort run data to be stored

in each extent

11-7 Copyright  Oracle Corporation, 1998 All rights reserved.

Guidelines for Temporary Segments

• Set up different TEMPORARY tablespaces

based on sorting needs

• Specify DEFAULT STORAGE for these

tablespaces as shown:

– INITIAL= NEXT = (multiple of

SORT_AREA_SIZE) + DB_BLOCK_SIZE

– PCTINCREASE = 0

• Specify a PCTINCREASE of zero to ensure that all extents are of the

same size

tablespace is a PERMANENT tablespace

Note

Create tablespaces with different default storage clauses and assign them to

users based on their sorting needs

Oracle8: Database Administration 11-11

Obtaining Temporary Segment Information

Obtaining Temporary Segment Information

The following views can be used to obtain information about temporary

segments and their usage:

This view only shows currently active sorts for the instance

11-8 Copyright  Oracle Corporation, 1998 All rights reserved.

Obtaining Information for a

Database Instance

DBA_SEGMENTS

V$SORT_SEGMENT

V$SORT_USAGE

Query the V$SORT_SEGMENT view to check the TEMPORARY

tablespaces containing sort segments and their usage statistics:

The columns MAX_SORT_SIZE and MAX_SORT_BLOCKS specify the

number of extents and blocks used by the largest sort operation that used a

11-9 Copyright  Oracle Corporation, 1998 All rights reserved.

Sort Segment Statistics

Oracle8: Database Administration 11-13

Obtaining Temporary Segment Information

Join V$SESSION and V$SORT_USAGE views to obtain information on the

currently active sorts in the instance:

Example

SVRMGR> SELECT s.username, u."USER", u.tablespace,

2> u.contents, u.extents, u.blocks

3> FROM v$session s,v$sort_usage u

The USER column in V$SORT_USAGE always shows the user querying

this view and not the user performing the sort Always obtain the user name

from the V$SESSION view The CONTENTS column shows whether the

temporary segment is created in a PERMANENT or a TEMPORARY

tablespace

11-10 Copyright  Oracle Corporation, 1998 All rights reserved.

Temporary Segment Activity

V$SESSION

SADDR USERNAME SID

V$SORT_USAGE

SESSION_ADDR TABLESPACE CONTENTS EXTENTS BLOCKS

Quick Reference

Initialization parameters DB_BLOCK_SIZE

Dynamic initialization parameters

(deferred)

SORT_AREA_SIZE

Dynamic performance views V$SORT_SEGMENT

V$SORT_USAGE V$SESSION Data dictionary views DBA_SEGMENTS

Commands CREATE TABLESPACE TEMPORARY

ALTER TABLESPACE TEMPORARY ALTER TABLESPACE PERMANENT

11-11 Copyright  Oracle Corporation, 1998 All rights reserved.