Oracle Bitmap Freelists

When you create a locally managed tablespace using the create TABLESPACE statement, the SEGMENT SPACE MANAGEMENT AUTO replaces the one-way linked list with a bitmap array freelists.

This clause lets you specify how free and used space within a segment is to be managed.  It completely eliminates any need to specify and tune the pctused, freelists, and freelist groups storage parameters for schema objects created in the tablespace. If any of these attributes are specified, they are ignored.

For example, the following statement creates tablespace mytbs1 with automatic segment-space management:

CREATE TABLESPACE mytbs1
DATAFILE '/u01/oracle/data/mytbs01.dbf' SIZE 500M
EXTENT MANAGEMENT LOCAL
SEGMENT SPACE MANAGEMENT AUTO;

Automatic segment-space management offers the following benefits:

• Segment space management auto provides administrative ease of use by avoiding the specification of storage parameters.
• Segment space management auto is a good method for handling objects with varying row sizes.
• Segment space management auto provides better run-time adjustment for variations in concurrent access and avoids tedious tuning methods.
• Segment space management auto provides better multi-instance behavior in terms of performance/space utilization.

The in-segment free/used space is tracked using bitmaps as opposed to the free lists. When you cannot use the locally managed tablespace, and therefore the automatic management space feature, you have to depend on the traditional method of managing free lists and free lists groups.

However, note that this automatic feature of segment space management is available only with locally managed tablespaces and their objects. A new column called SEGMENT_SPACE_MANAGEMENT has been added to the dba_tablespaces view to indicate the segment space management mode used by a tablespace.

Use the Oracle procedure dbms_space.space_usage to provide the space usage ratio within each block in the Bitmap Managed Block (BMB) segments.