Zurück

Migrate Dictionary managed Tablespaces to locally managed

Overview

Like any other object, the data dictionary manages space allocation to a tablespace. That means Oracle updates the appropriate tables in the data dictionary whenever an extent is allocated or freed for reuse. Oracle also stores rollback information about each update of the dictionary tables.

Because dictionary tables and rollback segments are part of the database, the space that they occupy is subject to the same space management operations as all other data.

This method of space management is called extent management by the data dictionary. These tablespaces are also called as Dictionary managed tablespaces. This was the only option available prior to release 8.1.

In Oracle release 8.1 and above you have an option to manage the space allocation called extent management by the tablespace.

Locally Managed Tablespaces

A tablespace that can manage extent allocation by itself is called locally managed tablespace. These tablespaces maintain a bitmap in each datafile to keep track of the freed or used status of blocks in that datafile. Each bit in the bitmap corresponds to a block or a group of blocks. When an extent is allocated or freed for reuse, Oracle changes the bitmap values to show the new status of the blocks. These changes do not generate rollback information because they do not update tables in the data dictionary (except for special cases such as tablespace quota information).

The storage parameters NEXT, PCTINCREASE, MINEXTENTS, MAXEXTENTS, and DEFAULT STORAGE are not valid for extents that are managed locally !

Extent Allocation

Locally managed tablespaces can have uniform extent sizes or variable extent sizes that are determined by the system. Any of the options, UNIFORM or AUTOALLOCATE can be mentioned while creating the tablespace. For UNIFORM extents you can specify an extent size. The default size is 1MB. For AUTOALLOCATE extents you can specify the size of the initial extent and Oracle determines the optimal size of the additional extents, with a minimum extent size of 64KB. That is why these are called system-managed extents.

How Extents are Allocated

Oracle looks for free space to allocate to a new extent by first determining a candidate datafile in the tablespace and then searching the datafile's bitmap for the required number of adjacent free blocks. If that datafile does not have enough adjacent free space, Oracle looks in another datafile. When extents are deallocated, Oracle modifies the bitmap in the datafile.

Create A Locally Managed Tablespace

CREATE TABLESPACE users
  DATAFILE 'users.dbf'
  EXTENT MANAGEMENT LOCAL
  UNIFORM SIZE 256K;

Advantages

  1. Local management of extents avoids recursive space management operations, which can occur in dictionary managed tablespaces.
     
  2. Local management of extents automatically tracks adjacent free space, eliminating the need to coalesce free extents.
     
  3. Reliance on data dictionary is reduced.

Notes

  1. Temporary tablespaces that manage their extents locally can only use UNIFORM extent allocation.
     
  2. For permanent tablespaces the default extent size for system managed extents is 64KB.
     
  3. The storage parameters NEXT, PCTINCREASE, MINEXTENTS, MAXEXTENTS, and DEFAULT STORAGE are not valid for locally managed tablespaces.

Change normal tablespaces to locally managed

Check extent management of the tablespaces with following command:

SQL> SELECT tablespace_name,extent_management
       FROM dba_tablespaces;

TABLESPACE_NAME                EXTENT_MAN
------------------------------ ----------
SYSTEM                         DICTIONARY
TEMP                           LOCAL
USERS                          DICTIONARY
TAB                            DICTIONARY
IDX                            DICTIONARY
SYSAUX                         LOCAL
UNDO                           LOCAL

First change first all dictionary managed tablespaces except tablespace SYSTEM to locally managed with the following procedure:

SQL> execute DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL ('USERS');
 PL/SQL procedure successfully completed.

The tablespace must be kept online and read/write during migration. Note that temporary tablespaces cannot be migrated.

Change SYSTEM tablespaces to locally managed

Before the SYSTEM tablespace can be migrated to locally managed format, you should ensure the following:

  • The database has a default temporary tablespace which is not SYSTEM

  • There are not any rollback segments in dictionary managed tablespaces

  • There is at least one online rollback segment in a locally managed tablespace, or an undo tablespace (if using automatic undo management mode) should be online.

  • All tablespaces other than the tablespace containing the undo space (undo tablespace or the tablespace containing the rollback segment) and the default temporary tablespace are in read-only mode.

  • There is a complete backup of the system.

  • The system is in restricted mode.

Notr, that we already have an UNDO Tablespace. The following query determines whether the SYSTEM tablespace is locally managed:

SQL> SELECT ts# FROM ts$ WHERE ts# = 0 AND bitmapped <> 0;

If 0 rows are returned, then the SYSTEM tablespace is dictionary managed. Otherwise, the SYSTEM tablespace is locally managed.

Steps to change SYSTEM tablespaces to locally managed

SQL> shutdown immediate
 SQL> startup restrict

SQL> execute DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL ('SYSTEM');

ERROR at line 1:
ORA-10644: SYSTEM tablespace cannot be default temporary tablespace
ORA-06512: at "SYS.DBMS_SPACE_ADMIN", line 227
ORA-06512: at line 1

SQL> ALTER DATABASE DEFAULT TEMPORARY TABLESPACE temp;

SQL> execute DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL ('SYSTEM');

ERROR at line 1:
ORA-10647: Tablespace other than SYSTEM, UNDO, TEMP not
found in read only mode
ORA-06512: at "SYS.DBMS_SPACE_ADMIN", line 227
ORA-06512: at line 1

SQL> select tablespace_name from dba_tablespaces;

TABLESPACE_NAME
------------------------------
SYSTEM
TEMP
USERS
TAB
IDX
SYSAUX
UNDO

SQL> alter tablespace USERS read only;
SQL> alter tablespace TAB read only;
SQL> alter tablespace IDX  read only;

SQL> execute DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL ('SYSTEM');

ERROR at line 1:
ORA-10648: Tablespace SYSAUX is not offline
ORA-06512: at "SYS.DBMS_SPACE_ADMIN", line 227
ORA-06512: at line 1

SQL> alter tablespace SYSAUX offline;

SQL> execute DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL ('SYSTEM');
PL/SQL procedure successfully completed.