14 Copyright © 2006, Oracle. All rights reserved. Tuning Block Space Usage.

14Copyright © 2006, Oracle. All rights reserved.

Tuning Block Space Usage

14-2 Copyright © 2006, Oracle. All rights reserved.

Objectives

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

• Tune segment space management

• Convert from dictionary-managed tablespaces

• Convert to Automatic Segment Space Management

• Tune block space management

• Diagnose and correct row migration

• Diagnose table fragmentation

• Compare characteristics of bigfile and smallfile tablespaces


Space Management

Space is managed at three levels:

• Files (OS, ASM, raw partitions) assign disk space to tablespaces.

• Extents are used to allocate file space to segments in a tablespace.

• Blocks are used to organize the space inside data objects.


Extent Management

Extents are allocated in two ways:

• Dictionary managed– Only supported for backward compatibility– Extents managed in the EXT$ and FET$ dictionary

tables – Recursive SQL

• Locally managed– Extents managed in the file header bitmap– No undo created on extent operations– Possible contention on file header blocks


Locally Managed Extents

• Create a locally managed tablespace:

• Default extent management is local.

SQL> CREATE TABLESPACE user_data_1

2 DATAFILE

3 ‘/oracle9i/oradata/db1/lm_1.dbf’

4 SIZE 100M

5 EXTENT MANAGEMENT LOCAL

6 UNIFORM SIZE 2M;


Pros and Cons of Large Extents

• Pros:– Are less likely to extend dynamically– Deliver a small performance benefit– Enable the server process to read the entire extent

map with a single I/O operation

• Cons:– Free space may not be available– May contain unused space


Migrating the SYSTEM Tablespaceto a Locally Managed Tablespace

• Use the DBMS_SPACE_ADMIN package:

• The locally managed SYSTEM tablespace restrictions are enforced by the migration procedure.

• Migration is possible only when the system is in RESTRICTED mode whereas all tablespaces other than SYSTEM, UNDO, and TEMP are in READ ONLY mode.

• Convert other dictionary-managed tablespaces with the same procedure before converting SYSTEM.

SQL> EXECUTE DBMS_SPACE_ADMIN. - 2 TABLESPACE_MIGRATE_TO_LOCAL('SYSTEM');


How Table Data Is Stored

Tablespace

Table A Table B

SegmentSegment

Rows

Columns

Table

Blocks

Row piece Extent


Anatomy of a Database Block

Block header

Free space

Row data

Growth


Minimize Block Visits

Minimize block visits by:

• Using a larger block size

• Packing rows tightly

• Preventing row migration

Tablespace

Blocks

Segments

Extents


The DB_BLOCK_SIZE Parameter

The database block size:

• Is defined by the DB_BLOCK_SIZE parameter

• Is set when the database is created

• Becomes the default block size for tablespaces and buffer cache

• Is the minimum I/O unit for data file reads

• Is 8 KB by default; up to 32 KB allowed on most platforms

• Cannot be changed easily

• Should be an integer multiple of the operating system (OS) block size

• Should be less than or equal to the OS I/O size


Small Block Size: Considerations

• Advantages:– Reduces block contention– Is good for small rows– Is good for random access

• Disadvantages:– Has a relatively large space overhead– Has a small number of rows per block– Can cause more index blocks to be read


Large Block Size: Considerations

• Advantages:– Less space overhead– Good for sequential access– Good for very large rows– Better performance of index reads

• Disadvantages:– Increases block contention– Uses more space in the buffer cache


Block Allocation

• When an INSERT or UPDATE operation requires more space, a block must be found with adequate space.

• Two methods: – Free lists– Automatic Segment Space Management (ASSM)


Free Lists

Free list–managed space characteristics:

• Segment header blocks hold free lists.

• Blocks are added to and removed from the free lists.

• Free lists are searched for available blocks.

• Segment headers are pinned for the search and update of free lists.


Block Space Management

Each segment has parameters that control the space usage inside a block. For a table:

• PCTFREE: Amount of space reserved for updates

• PCTUSED: A minimum level of free space in a block before a block is placed on the free list

For an index:

• PCTFREE: Amount of space reserved for new index entries at creation time

• PCTUSED: Always 0 for indexes


Block Space Management with Free Lists

Inserts

Inserts

Inserts

1

3

2

4

Inserts

PCTFREE

PCTUSED


Automatic Segment Space Management

Automatic Segment Space Management (ASSM) characteristics:

• Space is managed with bitmap blocks (BMB).

• Multiple processes search different BMBs.

• Availability of block is shown with a full bit.

• The fullness is shown by a percentage full bit for each of 25, 50, 75, and 100 percent used.


Automatic Segment Space Managementat Work

Extent

BMB

BMB BMB

BMB BMB

DATA

BMB

BMB

BMB BMB BMB

{SEGMENT

Block

… … … …

…

…

…

…

…

…

…BMB

BMB


Block Space Management with ASSM

Inserts

Inserts

Inserts

1

3

2

4

Inserts

25%

50%

75%


Creating an Automatic Segment SpaceManagement Segment

• SEGMENT SPACE MANAGEMENT is the attribute used for tablespace creation, which cannot be subsequently altered.

• Segment space management is declared at the tablespace level.

• Tablespace must be permanent and locally managed.

• Automatic space management segments are specified through the AUTO keyword.

• For free-list segments, use the default value of MANUAL.

• For ASSM, PCTUSED, FREELIST, and FREELIST GROUPS are ignored at table creation.


Migration and Chaining

Index Table

Migration

Chaining


Guidelines for PCTFREE and PCTUSED

• PCTFREE– Default: 10– Zero if no UPDATE activity

– PCTFREE = 100 × UPD / (average row length)

• PCTUSED– Only with free lists– Default: 40– Set if rows are deleted

– PCTUSED = 100 – PCTFREE – (100 × rows × average row length / block size)


Detecting Migration and Chaining

Use the ANALYZE command to detect migrationand chaining:

Statistic Total Per transaction ... ------------------------- ----- --------------- ...table fetch continued row 495 .02 …

SQL> ANALYZE TABLE oe.orders COMPUTE STATISTICS;Table Analyzed.SQL> SELECT num_rows, avg_row_len, chain_cnt 2 FROM DBA_TABLES 3 WHERE table_name='ORDERS'; NUM_ROWS AVG_ROW_LEN CHAIN_CNT---------- ----------- ---------- 1171 67 83

Detect migration and chaining by using Statspack/AWR:


Selecting Migrated Rows

SQL> ANALYZE TABLE oe.orders LIST CHAINED ROWS;

Table analyzed.

SQL> SELECT owner_name, table_name, head_rowid

2 FROM chained_rows

3 WHERE table_name = 'ORDERS';

OWNER_NAME TABLE_NAME HEAD_ROWID

---------- ---------- ------------------

SALES ORDER_HIST AAAAluAAHAAAAA1AAA

SALES ORDER_HIST AAAAluAAHAAAAA1AAB

...


Eliminating Migrated Rows

• Export/import:– Export the table.– Drop or truncate the table.– Import the table.

• MOVE table command:

– ALTER TABLE EMPLOYEES MOVE• Online table redefinition

• Copy migrated rows:– Find migrated rows by using ANALYZE. – Copy migrated rows to a new table.– Delete migrated rows from the original table.– Copy rows from the new table to the original table.


Shrinking Segments: Overview

HWM

HWM

Shrinkoperation

DataUnusedspace

Data

Unusedspace

Reclaimed space


Shrinking Segments: Considerations

• A shrink operation is an online and in-place operation.

• It is applicable only to segments residing in ASSM tablespaces.

• Candidate segment types:– Heap-organized tables and index-organized tables– Indexes– Partitions and subpartitions– Materialized views and materialized view logs

• Indexes are maintained.

• Triggers are not fired.


Shrinking Segments by Using SQL

ALTER … SHRINK SPACE [COMPACT][CASCADE]

TABLE [OVERFLOW] INDEX MATERIALIZED VIEW MATERIALIZED VIEW LOG

MODIFY PARTITION MODIFY SUBPARTITION MODIFY LOB

ALTER TABLE employees SHRINK SPACE CASCADE;

ALTER TABLE employees ENABLE ROW MOVEMENT; 1

2

ALTER TABLE employees MODIFY LOB(resume) (SHRINK SPACE);3

ALTER TABLE employees OVERFLOW SHRINK SPACE; 4


Segment Shrink: Basic Execution

HWM

ALTER TABLE employees SHRINK SPACE COMPACT;

HWM

HWM

ALTER TABLE employees SHRINK SPACE;

1

2


Segment Shrink:Execution Considerations

• Use compaction only:– To avoid unnecessary cursor invalidation– During peak hours

• DML operations and queries can be issued during compaction.

• DML operations are blocked when HWM is adjusted.


Using EM to Shrink Segments


Bigfile Tablespaces: Overview

• A bigfile tablespace contains a single file.

• Maximum file size ranges from 8 TB to 128 TB.

• Tablespaces are logically equivalent to data files.

Database

Tablespace

SMALLFILEDatafile

BIGFILEDatafile


Bigfile Tablespaces: Benefits

• Significantly increases the storage capacity

• Simplifies data file management for large databases by making tablespaces the main units of disk space administration

BFT 1 …

8 EB4 billionblocks

One-to-one mapping

BFT n


Using Bigfile Tablespaces

• Supported only for locally managed tablespaces using Automatic Segment Space Management

• Use with logical volume managers or Automatic Storage Management (ASM)

• OMF used for complete data file transparency

800,00016 KB

400,0008 KB

200,0004 KB

100,0002 KB

Recommended Maximum Number of Extents

Database Block Size


Practice Overview: Tune Database Space Usage

This practice covers the following topics:

• Convert from dictionary-managed to locally managed tablespaces

• Diagnose free-list contention

• Convert segments from Manual to Automatic Segment Space Management


Summary

In this lesson, you should have learned how to:

• Tune segment space management

• Convert from dictionary-managed tablespaces

• Convert to Automatic Segment Space Management

• Tune block space management

• Diagnose and correct row migration

• Diagnose table fragmentation

• Compare characteristics of bigfile and smallfile tablespaces

14 Copyright © 2006, Oracle. All rights reserved. Tuning Block Space Usage.

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