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Oracle Managed Files (OMF)

Oracle Tips by Burleson Consulting

Oracle Managed Files (OMF) directly manages file creation and deletion at the operating system level. Operations are specified in terms of database objects rather than filenames. Oracle internally uses standard file system interfaces to create and delete files as needed for the database structures such as tablespaces, online redo log files, and control files.

Administrators need to specify the file system directory to be used for a particular type of file. Thereafter, Oracle manages to create unique files when needed and also deletes files that are not needed. The advantages include:

* No need to bother about the file names and storage requirements. Oracle provides a unique file name.

* Eliminates the need for the DBA to directly manage OS files.

* Allows operations to be specified in terms of objects and not files.

* Reduces the chance of overwriting a file. Avoids accidental corruption of data files.

* Reduces wasted disk space consumed by obsolete files. When a tablespace is dropped, the relevant files are automatically removed at the host level.

The following initialization parameters specify the file location:

* db_create_file_dest: Defines the location of the default file system directory where Oracle creates data files or temp files when no file specification is given in the object creation statement.

* db_create_online_log_dest_n: Defines the location of the default file system directory for online redo log files and control files when no file specification is given in the creation statement. This initialization parameter can be used multiple times where n specifies a multiplexed copy of the online redo log or control file.

Note that the file system directory specified by either of these parameters must already exist. Oracle does not create it. The directory must also have permissions to allow Oracle to create the files in it. The default location is used whenever a location is not explicitly specified for the operation of creating a file. Oracle creates the filename, and a file thus created is an Oracle-managed file. The following example sets /u01/oracle/data/applic as the default directory to use when creating Oracle-managed files:

DB_CREATE_FILE_DEST = '/u01/oracle/data/applic'

By default, this location would be applicable for data files, temp files, Online redo log files, and control files. However, the db_create_online_log_dest_n initialization parameter can be included in the initialization parameter file to identify the default location for the database server to create online redoes log files and control files. For example:

DB_CREATE_ONLINE_LOG_DEST_1 = '/u03/oracle/redo/applic'
DB_CREATE_ONLINE_LOG_DEST_2 = '/u04/oracle/redo/applic'

Also, the DB_CREATE location can also be changed with the ALTER SYSTEM command.

For example, the following commands change the OMF directory location to '/u05/oracle/data/applic', and creates a new tablespace.

ALTER SYSTEM SET
DB_CREATE_FILE_DEST = '/u05/oracle/data/applic' ; 

CREATE TABLESPACE tbs_1; # creates def.size of 100MB
CREATE TABLESPACE tbs_3
DATAFILE AUTOEXTEND ON MAXSIZE 800M;

For more details, refer to the chapter on Using Oracle Managed Files in the Oracle 10g Database Administrator's Guide.

With RAC systems, the OMF facility can only be used if the cluster system is using the cluster file system. The availability of the cluster file system with a set of mounted file systems, allows a directory to be set up for OMF files. The DBCA also supports Oracle Managed Files for operating systems that support a cluster file system.

   
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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