Managing Tablespaces and Datafiles


Managing Tablespaces and Datafiles

Using multiple tablespaces provides several Advantages
  • Separate user data from data dictionary data to reduce contention among dictionary objects and schema objects for the same datafiles.
  • Separate data of one application from the data of another to prevent multiple applications from being affected if a tablespace must be taken offline.
  • Store different the datafiles of different tablespaces on different disk drives to reduce I/O contention.
  • Take individual tablespaces offline while others remain online, providing better overall availability.

Creating New Tablespaces

You can create Locally Managed or Dictionary Managed Tablespaces. In prior versions of Oracle only Dictionary managed Tablespaces were available but from Oracle ver. 8i you can also create Locally Managed tablespaces. The advantages of locally managed tablespaces are
Locally managed tablespaces track all extent information in the tablespace itself by using bitmaps, resulting in the following benefits:
  • Concurrency and speed of space operations is improved, because space allocations and deallocations modify locally managed resources (bitmaps stored in header files) rather than requiring centrally managed resources such as enqueues
  • Performance is improved, because recursive operations that are sometimes required during dictionary-managed space allocation are eliminated
To create a locally managed tablespace give the following command

SQL> CREATE TABLESPACE ica_lmts DATAFILE '/u02/oracle/ica/ica01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL AUTOALLOCATE;

AUTOALLOCATE causes the tablespace to be system managed with a minimum extent size of 64K.

The alternative to AUTOALLOCATE is UNIFORM. which specifies that the tablespace is managed with extents of uniform size. You can specify that size in the SIZE clause of UNIFORM. If you omit SIZE, then the default size is 1M. The following example creates a Locally managed tablespace with uniform extent size of 256K

SQL> CREATE TABLESPACE ica_lmt DATAFILE '/u02/oracle/ica/ica01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 256K;

To Create Dictionary Managed Tablespace
SQL> CREATE TABLESPACE ica_lmt DATAFILE '/u02/oracle/ica/ica01.dbf' SIZE 50M EXTENT MANAGEMENT DICTIONARY;

Bigfile Tablespaces (Introduced in Oracle Ver. 10g)
A bigfile tablespace is a tablespace with a single, but very large (up to 4G blocks) datafile. Traditional small file tablespaces, in contrast, can contain multiple datafiles, but the files cannot be as large. Bigfile tablespaces can reduce the number of datafiles needed for a database.

To create a bigfile tablespace give the following command

SQL> CREATE BIGFILE TABLESPACE ica_bigtbs DATAFILE '/u02/oracle/ica/bigtbs01.dbf' SIZE 50G;

To Extend the Size of a tablespace

Option 1

 You can extend the size of a tablespace by increasing the size of an existing datafile by typing the following command
SQL> alter  database ica datafile ‘/u01/oracle/data/icatbs01.dbf’ resize 100M;
This will increase the size from 50M to 100M

Option 2

You can also extend the size of a tablespace by adding a new datafile to a tablespace. This is useful if the size of existing datafile is reached o/s file size limit or the drive where the file is existing does not have free space. To add a new datafile to an existing tablespace give the following command.
SQL> alter tablespace add datafile  ‘/u02/oracle/ica/icatbs02.dbf’ size 50M;

Option 3

You can also use auto extend feature of datafile. In this, Oracle will automatically increase the size of a datafile whenever space is required. You can specify by how much size the file should increase and Maximum size to which it should extend.
To make a existing datafile auto extendable give the following command
SQL> alter database datafile ‘/u01/oracle/ica/icatbs01.dbf’ auto extend ON next 5M maxsize 500M;
You can also make a datafile auto extendable while creating a new tablespace itself by giving the following command.
SQL> create tablespace ica datafile ‘/u01/oracle/ica/icatbs01.dbf’ size 50M auto extend ON next 5M maxsize 500M;

To decrease the size of a tablespace

You can decrease the size of tablespace by decreasing the datafile associated with it. You decrease a datafile only up to size of empty space in it. To decrease the size of a datafile give the following command
SQL> alter database datafile ‘/u01/oracle/ica/icatbs01.dbf’ resize 30M;

Coalescing Tablespaces

A free extent in a dictionary-managed tablespace is made up of a collection of contiguous free blocks. When allocating new extents to a tablespace segment, the database uses the free extent closest in size to the required extent. In some cases, when segments are dropped, their extents are deallocated and marked as free, but adjacent free extents are not immediately recombined into larger free extents. The result is fragmentation that makes allocation of larger extents more difficult.

You should often use the ALTER TABLESPACE ... COALESCE statement to manually coalesce any adjacent free extents. To Coalesce a tablespace give the following command

SQL> alter tablespace ica coalesce;





Ref: http://www.oracle-dba-online.com


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