1. 14 Copyright © 2006, Oracle. All rights reserved. Tuning Block Space Usage

2. 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

3. 14-3 Copyright © 2006, Oracle. All rights reserved. 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.

4. 14-4 Copyright © 2006, Oracle. All rights reserved. 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

5. 14-5 Copyright © 2006, Oracle. All rights reserved. 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;

6. 14-6 Copyright © 2006, Oracle. All rights reserved. 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

7. 14-7 Copyright © 2006, Oracle. All rights reserved.

8. 14-8 Copyright © 2006, Oracle. All rights reserved. Migrating the SYSTEM Tablespace to 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');

9. 14-9 Copyright © 2006, Oracle. All rights reserved.

10. 14-10 Copyright © 2006, Oracle. All rights reserved. How Table Data Is Stored Tablespace Table ATable B Segment Rows Columns Table Blocks Row piece Extent

11. 14-11 Copyright © 2006, Oracle. All rights reserved. Anatomy of a Database Block Block header Free space Row data Growth

12. 14-12 Copyright © 2006, Oracle. All rights reserved. Minimize Block Visits Minimize block visits by: Using a larger block size Packing rows tightly Preventing row migration Tablespace Blocks Segments Extents

13. 14-13 Copyright © 2006, Oracle. All rights reserved. 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

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

15. 14-15 Copyright © 2006, Oracle. All rights reserved. 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

16. 14-16 Copyright © 2006, Oracle. All rights reserved. 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)

17. 14-17 Copyright © 2006, Oracle. All rights reserved. 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.

18. 14-18 Copyright © 2006, Oracle. All rights reserved. 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

19. 14-19 Copyright © 2006, Oracle. All rights reserved. Block Space Management with Free Lists Inserts 1 3 2 4 PCTFREE PCTUSED

20. 14-20 Copyright © 2006, Oracle. All rights reserved.

21. 14-21 Copyright © 2006, Oracle. All rights reserved. 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.

22. 14-22 Copyright © 2006, Oracle. All rights reserved. Automatic Segment Space Management at Work Extent BMB DATA BMB { SEGMENTSEGMENT Block ……… … … … … … … … … BMB

23. 14-23 Copyright © 2006, Oracle. All rights reserved.

24. 14-24 Copyright © 2006, Oracle. All rights reserved. Block Space Management with ASSM Inserts 1 3 2 4 25% 50% 75%

25. 14-25 Copyright © 2006, Oracle. All rights reserved. Creating an Automatic Segment Space Management 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.

26. 14-26 Copyright © 2006, Oracle. All rights reserved. Migration and Chaining IndexTable Migration Chaining

27. 14-27 Copyright © 2006, Oracle. All rights reserved.

28. 14-28 Copyright © 2006, Oracle. All rights reserved. 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)

29. 14-29 Copyright © 2006, Oracle. All rights reserved. Detecting Migration and Chaining Use the ANALYZE command to detect migration and 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:

30. 14-30 Copyright © 2006, Oracle. All rights reserved. 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...

31. 14-31 Copyright © 2006, Oracle. All rights reserved. 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.

32. 14-32 Copyright © 2006, Oracle. All rights reserved.

33. 14-33 Copyright © 2006, Oracle. All rights reserved. Shrinking Segments: Overview HWM Shrink operation Data Unused space Data Unused space Reclaimed space

34. 14-34 Copyright © 2006, Oracle. All rights reserved. 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.

35. 14-35 Copyright © 2006, Oracle. All rights reserved. Shrinking Segments by Using SQL ALTER … SHRINK SPACE [COMPACT][CASCADE] TABLE [OVERFLOW]INDEXMATERIALIZED VIEWMATERIALIZED VIEW LOG MODIFY PARTITIONMODIFY 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

36. 14-36 Copyright © 2006, Oracle. All rights reserved. Segment Shrink: Basic Execution HWM ALTER TABLE employees SHRINK SPACE COMPACT; HWM ALTER TABLE employees SHRINK SPACE; 1 2

37. 14-37 Copyright © 2006, Oracle. All rights reserved. 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.

38. 14-38 Copyright © 2006, Oracle. All rights reserved. Using EM to Shrink Segments

39. 14-39 Copyright © 2006, Oracle. All rights reserved. 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 SMALLFILE Data file BIGFILE Data file

41. 14-41 Copyright © 2006, Oracle. All rights reserved. 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 EB 4 billion blocks One-to-one mapping BFT n

42. 14-42 Copyright © 2006, Oracle. All rights reserved. Full Notes Page

43. 14-43 Copyright © 2006, Oracle. All rights reserved. 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

44. 14-44 Copyright © 2006, Oracle. All rights reserved. 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

45. 14-45 Copyright © 2006, Oracle. All rights reserved. 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

46. 14-46 Copyright © 2006, Oracle. All rights reserved.