Information Technology Auditor (PA) Next Step Systems US-PA-Wayne Post A Job | Post A Resume

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).

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 tbs1
DATAFILE 'file1.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.

DBAsupport.com Home Page