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Oracle ASSM

Oracle Tips by Burleson Consulting
Don Burleson

The following is from the bestselling book Oracle 10g Grid & Real Application Clusters.

 


Automatic segment space management (ASSM) is a simpler and more efficient way of managing space within a segment. 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.

See here for ASSM internals and how bitmap freelists compare to manual freelist management.

When you create a locally managed tablespace using the create TABLESPACE statement, the SEGMENT SPACE MANAGEMENT clause lets you specify how free and used space within a segment is to be managed.

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;

When an object such as a table or index is created using the locally managed tablespace, with automatic segment-space management enabled, there is no need to specify the pctused or freelists.

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.

PCTFREE is still required even with ASSM because it determines how rows will be packed into blocks, while freelists, pctused and pctincrease are ignored with ASSM because they are used for transaction/block management.


 

 

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