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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.

SQL> select tablespace_name, extent_management, allocation_type from dba_tablespaces;

TABLESPACE_NAME                EXTENT_MAN ALLOCATIO
------------------------------ ---------- ---------
SYSTEM                         DICTIONARY USER
SYS_UNDOTS                     LOCAL      SYSTEM
TEMP                           LOCAL      UNIFORM

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;

Note the difference between AUTOALLOCATE and UNIFORM SIZE:

AUTOALLOCATE specifies that extent sizes are system managed. Oracle will choose "optimal" next extent sizes starting with 64KB. As the segment grows larger extent sizes will increase to 1MB, 8MB, and eventually to 64MB. This is the recommended option for a low or unmanaged environment.

UNIFORM specifies that the tablespace is managed with uniform extents of SIZE bytes (use K or M to specify the extent size in kilobytes or megabytes). The default size is 1M. The uniform extent size of a locally managed tablespace cannot be overridden when a schema object, such as a table or an index, is created.

Also not, if you specify, LOCAL, you cannot specify DEFAULT STORAGE, MINIMUM EXTENT or TEMPORARY.

Advantages of Locally Managed Tablespaces:

  • Eliminates the need for recursive SQL operations against the data dictionary (UET$ and FET$ tables)
  • Reduce contention on data dictionary tables (single ST enqueue)
  • Locally managed tablespaces eliminate the need to periodically coalesce free space (automatically tracks adjacent free space)
  • Changes to the extent bitmaps do not generate rollback information

Locally Managed SYSTEM Tablespace:

From Oracle9i release 9.2 one can change the SYSTEM tablespace to locally managed. Further, if you create a database with DBCA (Database Configuration Assistant), it will have a locally managed SYSTEM tablespace by default. The following restrictions apply:

  • No dictionary-managed tablespace in the database can be READ WRITE.
  • You cannot create new dictionary managed tablespaces
  • You cannot convert any dictionary managed tablespaces to local

Thus, it is best only to convert the SYSTEM tablespace to LMT after
all other tablespaces are migrated to LMT.

Segment Space Management in LMT:

From Oracle 9i, one can not only have bitmap managed tablespaces, but also bitmap managed segments when setting Segment Space Management to AUTO for a tablespace. Look at this example:

SQL> CREATE TABLESPACE ts4 DATAFILE '/oradata/ts4_01.dbf' SIZE 50M 
      EXTENT MANAGEMENT LOCAL 
      SEGMENT SPACE MANAGEMENT AUTO;

Segment Space Management eliminates the need to specify and tune the PCTUSED, FREELISTS, and FREELISTS GROUPS storage parameters for schema objects. The Automatic Segment Space Management feature improves the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously eliminating serialization for free space lookups against the FREELSITS. This is of particular importance when using RAC, or if "buffer busy waits" are deteted.

Convert between LMT and DMT:

The DBMS_SPACE_ADMIN package allows DBAs to quickly and easily
convert between LMT and DMT mode. Look at these examples:

SQL> exec dbms_space_admin.Tablespace_Migrate_TO_Local('ts1');
PL/SQL procedure successfully completed.

SQL> exec dbms_space_admin.Tablespace_Migrate_FROM_Local('ts2');
PL/SQL procedure successfully completed.

This article is very much informative.

It is possible to have a Locally Managed SYSTEM tablespace. However, is it possible to set Segment Space Management Auto clause to the SYSTEM tablespace?

Thanks,
Chandran

Oracle version 9i rel 2 supports locally managed system tablespaces.

No, it is not possible to use

No, it is not possible to use ASSM for the SYSTEM tablespace (at least in Oracle 10g and 11g).

http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/statements_7003.htm

(I was searching the answer for the same question when I found your question first, and so the reply even if it is late)

Is it possibile to convert a

Is it possibile to convert a Dictionary managed Temporary tablespace to locally managed temporary tablespace?
exec dbms_space_admin.Tablespace_Migrate_TO_Local('temp');
or
Create a locally managed temp tablespace and drop the existing one.
Which one is your suggestion?

Locally vs dictionary

Excellent article. I was looking for a good explaination.

I now have to see how it works with RAC or ASM.
There is little information on that subject.

Richard.

LMT vs DMT

This is really a good article that portrays how LMT scores over DMT.

Thanks,
Sundar.N

Good One

This is excellant document for convertion of tablespaces.

Does Designer 10g support LMT/ ASSM?

Does Designer 10g support LMT/ ASSM? It looks like to me that initial, next etc still have to be specified in designer.

Excellent explanation

I have been looking for the difference between the two and I found this article from google. I think this is the best explanation I have read and very easy to understand the difference between the locally managed and dictionary managed tablespaces. Thanks for the article.

DMT to LMT

how wil u convert system tablespace from dictionary to local in 9.2.0.3?

i tried by using package DBMS_SPACE_ADMIBN.TABLESPACE_MIGRATE_TO_LOCAL('SYSTEM');

Good Article

Good Article

WHY CANNOT

Why we cannot have DMT in database having LM System Tablespace.
Does LM system not have sys.fet$ and uet$ tables?? or what is the reason.

Very Well Explained Article

Dictionary Managed Tablespaces and Locally Managed SYSTEM Tablespace is very well explained.There are very few articles explaining this topic with such a simplicity.