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Don Burleson Blog

Oracle automatic segment space management

Oracle Database Tips by Donald Burleson

Automatic segment space management (ASSM, or bitmap freelists) 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.

Beware: Using ASSM can hinder database DML performance, and most Oracle experts will use manual freelists and freelist groups. Click here for details on automatic segment space management internals.

Using automatic segment space management moves the freelists from one-way linked lists to bitmaps within the tablespace segment management blocks, allowing for higher concurrency (up to a point where multiple traditional freelists are better).

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

Using automatic segment space management is great for almost all Oracle applications, with the exception being super-high update tablespaces, which would use traditional freelists with multiple freelist groups.

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