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.
