Migrating Dictionary Managed Tablespace To Locally Managed Tablespace

 Migrate a dictionary managed SYSTEM tablespace to locally managed :

SQL> DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL          ('SYSTEM');

Use the below following steps to convert your SYSTEM tablespace from Dictionary Managed to locally Managed :

• Make a complete backup of your Database
• Ensure that the database has a default temporary tablespace that is not SYSTEM. The temporary tablespace is created using the CREATE TEMPORARY TABLESPACE command.
• Eliminate any undo (rollback) segments in disctionary-managed tablespaces.
• There should be at least one online undo segment in a locally managed tablespace, or an undo tablespace should be online.
• All tablespaces other than the tablespace containing the undo space and the default temporary tablespace should be placed in READ ONLY mode.
• Startup the instance in restricted mode.
• Migrate the SYSTEM tablespace by using the above command.

Note: Any non-SYSTEM dictionary managed tablespace that is not migrated to locally managed prior to migrating the SYSTEM tablespace, cannot be altered to READ-WRITE. The tablespace will remain READ-ONLY. Only locally managed tablespaces can be altered to READ-WRITE after the migration of the system tablespace.

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Space Management in Tablespace

Tablespaces allocated space in extents. Tablespaces can be created to use one of the following two different methods of keeping track of free and used space.

• Locally Managed tablespace
• Dictionary Managed Tablespace

Locally Managed tablespace:

- Free Extents are managed in the tablespace.

- Bitmap is used to records free extents.

- Each bit corressponds to a block or group of blocks.

- Bit Value indicates free or used.

The Extents are managed within the tablespace via bitmaps. Each bit in the bitmap corresponds to a block or a group of blocks. When an extent is allocated or freed for reuse, the oracle server changes the bitmap values to show the new status of the bocks. Locally managed is the default tablespace management it is introduced in Oracle 9i. 

Locally managed tablespaces are :

• Reduced contention on data dictionary tables.
• No undo generated when space allocation or deallocation occurs
• No Coalescing required.

SQL Command for Creation of locally managed tablespace for Example:

SQL> CREATE TABLESPACE userdata 

DATAFILE '/u01/oradata/userdata01.dbf' size 100M

EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K; 

The LOCAL option of the EXTENT MANAGEMENT clause specifies that a tablespace is to be locally managed. By default, a tablespace is locally managed.   

extent_management_clause :

 [EXTENT MANAGEMENT [ DISCTIONARY | LOCAL 

[AUTOALLOCATE | UNIFORM [SIZE integer [K/M]]]]

Where : 

DICTIONARY: Specifies that the tablespace is managed using dictionary tables.

LOCAL: Specifies that the tablespace is Locally managed via bitmaps. If you specify LOCAL, you cannot specify DEFAULT  storage_caluse, MINIMUM EXTENT, or TEMPORARY.

AUTOALLOCATE: Specifies that the tablespace is system managed. Users cannot specify the extent size. This is the default.

UNIFORM: Specifies that the tablespace is managed with uniform extents of size bytes. Use the K or M to specify the extent size in kilobytes or megabytes. The default size is 1 MB. 

The EXTENT_MANAGEMENT clause can be used in various CREATE command :

• For a permanent tablespace, you can specify EXTENT_MANAGEMENT_LOCAL in the CREATE TABLESPACE command. 

Note : Parior to Oracle 9i release 2 , the SYSTEM tablespace was not LOCALLY managed.

• For a temporary tablespace, you can specify EXTENT_MANAGEMENT_LOCAL in the CREATE TEMPORARY TABLESPACE  command.

 

Advantages of Locally Managed Tablespaces :

A locally managed tablespace has the following advantage  over the Dictionary Managed tablespaces :

• Local Management avoids recursive space management operations. This can occur in Dictionary-Managed tablespaces if consuming or releasing space to an extent and in results of this operation another operation that consumes or release space in an Undo segment or Data Dictionary tables.
• Because locally managed tablespaces do not record free space in the data dictionary tables, they reduce contention on these tables.
• Local management of extents automatically tracks adjacent free space, Eliminating the need to coalesce free extents.
• The sizes of the extents that are managed locally can be determined automatically by the system.
• Changes to the extent bitmaps do not generate undo information because they do not update tables in the data dictionary (except for special cases such as tablespace quota information). 

   

Dictionary- Managed Tablespace: Characteristics : 

- Free Extents are managed by the database disctionary.

- Extents are managed in the data dictionary.

- Appropriate tables are updated when extents are allocated or deallocated.

- Each segment stored in the tablespace can have a different storage clause.

- Coalescing is required.

The extents are managed by the data dictionary. The Oracle server updates the appropriate tables in the data dictionary whenever an extent is allocated or deallocated.

Segments in dictionary-managed tablespaces can have customized storage. This storage is more flexible than locally managed tablespaces but much less efficient.

SQL Command for Creation of Dictionary managed tablespace for Example:

SQL> CREATE TABLESPACE userdata

 DATAFILE  '/u01/oradata/userdata01.dbf' size 100M 

 EXTENT MANAGEMENT DICTIONARY DEFAULT STORAGE 

 (initial 1M next 1M PCTINCREASE 0);

 

Thank you !!

Managing Tablespaces and Data Files

 Tablespace and Data Files : 

Oracle Stores Data logically in tablespaces and Physically in Data files.

• Tablespaces :

- Can belong to only one databses at a time.

- Consist of one or more data files

- Are further devided into logical units of stoarge.

• Data Files :

- Can belong to only one tablespace and one databases

- Are a repository for Schema object data.

Databases, Tablespaces, and Data files are closely related, but they have important differences :

• An Oracle Database consists of one or more logical storage units called tablespaces,  which collectively store all of the database's data.
• Each tablespace in an Oracle Database consists of one or more files called data files, which are physical structures that conform with the operating system in which Oracle is running.
• A database's data is collectively stored in the data files that constitute each tablespace of the database. 

For example, the simplest Oracle Database would have one tablespace and one data file. Another database can have more tablespaces with more data files.

Types of tablespaces :

• SYSTEM  Tablespace

- Created with the database

- Required in all databases  

- Contains the data Dictionary, Including stored program units

- Contains the SYSTEM undo segments

- Should not contain user data, although it is allowed. 

• Non-SYSTEM tablespace

- Separate Segments

- Separate undo temporary, application data, and application index segments 

- Separate Data by backup requirements

- Separate dynamic and static data 

- Eases Space adminstration

- Controls amount of space allocation to a user

The DBA creates tablespace for increased control and ease of maintenance.  

Creating Tablespaces:

A tablespace is created using the command:  CREATE TABLESPACE:

Example:

CREATE TABLEPSACE userdata  DATAFILE 

'/u01/oradata/userdata01.dbf' size 100m;

Tablespace Creation syntax with more option Details :

Syntax: 

CREATE TABLESPACE tablespace name

[DATAFILE  clause] [MINIMUM EXTENT integer [K|M]]

[BLOCKSIZE integer [K]] [LOGGING | NOLOGGING]

[DEFAULT storage_clause] [ONLINE | OFFLINE ]

[PERMANENT | TEMPORARY] 

[extent_management_clause] [ segment_management_clause]

Where :

TABLESPACE NAME: This is the name of the tablespace to be created.

DATAFILE: This specifies the data file Or Data files name that uses to make up tablespace.

MINIMUM EXTENT: This ensures that every used extent size in the tablespace is a multiple of the integer. Use K or M to specify this size in Kilobytes or Megabytes.

BLOCKSIZE: Specifies a nonstandard block size for the tablespace. In order to specify this clause, you must have the DB_CACHE_SIZE and at least one DB_nK_CACHE_SIZE parameter set and the integer you specify in this clause must correspond with the setting of one DB_nK_CACHE_SIZE parameter setting.

LOGGING: This specifies that, by default, all tables, indexes, and partition within the tablespace have all changes written to online redo log files. LOGGING is the by default.

NOLOGGING: This specifies that, by default, all tables, indexes, and partition within the tablespace do not have all changes written to the online redo log file. NOLOGGING affects only some DML and DDL commands, For Example, Direct loads.

DEFAULT: DEFAULT clause specifies the default storage parameter for all objects created in the tablespace creation. 

OFFLINE: This clause makes the tablespace unavailable immediately after creation.

PERMANENT: This specifies that the tablespace can be used to hold permanent objects.

TEMPORARY: This specifies that the tablespace can be used to hold temporary objects. For example, segments used by implicit sorts caused by an ORDER BY clause. It cannot specify EXTENT MANAGEMENT LOCAL or the BLOCKSIZE clause.

extent_management_clause: This clause specifies how the extents of the tablespace are managed.

segment_management_caluse: This is relevant only for permanent, Locally managed tablespaces. It lets you specify whether Oracle should track the used and free space in the segments in the tablespace using free lists or bitmaps.

  Review below syntax:

datafile_clause:= file_name [SIZE integer (K|M)] [REUSE] [autoextend_caluse] [NEXT]

Filename : this is the name of a data file in the tablespace.

SIZE : This specifies the size if the file. Use K or M to specify the size in Kilobytes or Megabytes 

REUSE: This allows the Oracle server to reuse and existing file.

autoextend_clause : This Clause enables or disables automatic extention of the data file.

NEXT: This Specifies the size in bytes of the next increment of disk space to be allocated automatically when more extents are required.

Where:

MAXSIZE : this specifies the maximum disk space allowed for automatic extention of the datafile.

UNLIMITED :  This specifies the disk space that can be allocated to the data file, or that the tempfile is not limited. 

Thank you !!