Locally vs. Dictionary Managed Tablespaces

 When Oracle allocates space to a segment (like a table or index), a group of contiguous free blocks, called an extent, is added to the segment. Metadata regarding extent allocation and unallocated extents are either stored in the data dictionary, or in the tablespace itself. Tablespaces that record extent allocation in the dictionary, are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header, are called locally managed tablespaces.


Dictionary Managed Tablespaces (DMT):

Oracle use the data dictionary (tables in the SYS schema) to track allocated and free extents for tablespaces that is in "dictionary managed" mode. Free space is recorded in the SYS.FET$ table, and used space in the SYS.UET$ table. Whenever space is required in one of these tablespaces, the ST (space transaction) enqueue latch must be obtained to do inserts and deletes agianst these tables. As only one process can acquire the ST enque at a given time, this often lead to contention.
Execute the following statement to create a dictionary managed
tablespace:

SQL> CREATE TABLESPACE ts1 DATAFILE '/oradata/ts1_01.dbf' SIZE 50M 
      EXTENT MANAGEMENT DICTIONARY 
      DEFAULT STORAGE ( INITIAL 50K NEXT 50K MINEXTENTS 2 MAXEXTENTS 50 PCTINCREASE 0);

Locally Managed Tablespaces (LMT):

Using LMT, each tablespace manages it's own free and used space within a bitmap structure stored in one of the tablespace's data files. Each bit corresponds to a database block or group of blocks. Execute one of the following statements to create a locally managed
tablespace:
SQL> CREATE TABLESPACE ts2 DATAFILE '/oradata/ts2_01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL AUTOALLOCATE; SQL> CREATE TABLESPACE ts3 DATAFILE '/oradata/ts3_01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K;