Locally vs. Dictionary Managed Tablespaces

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     Locally vs. Dictionary Managed Tablespaces



When Oracle allocates space to a segment (like a table or index), a group of contiguous free blocks, called an extent, is added to the segment. Metadata regarding extent allocation and unallocated extents are either stored in the data dictionary, or in the tablespace itself. Tablespaces that record extent allocation in the dictionary, are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header, are called locally managed tablespaces.
SQL> select tablespace_name, extent_management, allocation_type from dba_tablespaces;

TABLESPACE_NAME                EXTENT_MAN ALLOCATIO
------------------------------ ---------- ---------
SYSTEM                         DICTIONARY USER
SYS_UNDOTS                     LOCAL      SYSTEM
TEMP                           LOCAL      UNIFORM
Dictionary Managed Tablespaces (DMT):
Oracle use the data dictionary (tables in the SYS schema) to track allocated and free extents for tablespaces that is in "dictionary managed" mode. Free space is recorded in the SYS.FET$ table, and used space in the SYS.UET$ table. Whenever space is required in one of these tablespaces, the ST (space transaction) enqueue latch must be obtained to do inserts and deletes agianst these tables. As only one process can acquire the ST enque at a given time, this often lead to contention.
Execute the following statement to create a dictionary managed
tablespace:
SQL> CREATE TABLESPACE ts1 DATAFILE '/oradata/ts1_01.dbf' SIZE 50M 
      EXTENT MANAGEMENT DICTIONARY 
      DEFAULT STORAGE ( INITIAL 50K NEXT 50K MINEXTENTS 2 MAXEXTENTS 50 PCTINCREASE 0);
Locally Managed Tablespaces (LMT):
Using LMT, each tablespace manages it's own free and used space within a bitmap structure stored in one of the tablespace's data files. Each bit corresponds to a database block or group of blocks. Execute one of the following statements to create a locally managed
tablespace:
SQL> CREATE TABLESPACE ts2 DATAFILE '/oradata/ts2_01.dbf' SIZE 50M
    EXTENT MANAGEMENT LOCAL AUTOALLOCATE;

SQL> CREATE TABLESPACE ts3 DATAFILE '/oradata/ts3_01.dbf' SIZE 50M
      EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K;
Note the difference between AUTOALLOCATE and UNIFORM SIZE:
AUTOALLOCATE specifies that extent sizes are system managed. Oracle will choose "optimal" next extent sizes starting with 64KB. As the segment grows larger extent sizes will increase to 1MB, 8MB, and eventually to 64MB. This is the recommended option for a low or unmanaged environment.
UNIFORM specifies that the tablespace is managed with uniform extents of SIZE bytes (use K or M to specify the extent size in kilobytes or megabytes). The default size is 1M. The uniform extent size of a locally managed tablespace cannot be overridden when a schema object, such as a table or an index, is created.
Also not, if you specify, LOCAL, you cannot specify DEFAULT STORAGE, MINIMUM EXTENT or TEMPORARY.
Advantages of Locally Managed Tablespaces: 
  • Eliminates the need for recursive SQL operations against the data dictionary (UET$ and FET$ tables)
  • Reduce contention on data dictionary tables (single ST enqueue)
  • Locally managed tablespaces eliminate the need to periodically coalesce free space (automatically tracks adjacent free space)
  • Changes to the extent bitmaps do not generate rollback information



Maintenance

The DBMS_SPACE_ADMIN package contains a number of maintenance routines for locally managed tablespaces, including routines to convert from local to dictionary management. From Oracle 8.1.6 onwards the conversion from dictionary to local is also possible.
-- Local to dictionary
EXEC DBMS_SPACE_ADMIN.tablespace_migrate_from_local('TSH_DATA_3');

-- Dictionary to local
EXEC DBMS_SPACE_ADMIN.tablespace_migrate_to_local('TSH_DATA_3');
DONT FORGET TO COMMENT !!

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