10
Managing Datafiles

This chapter describes the various aspects of datafile management, and includes the following topics:

• Guidelines for Managing Datafiles

• Creating and Adding Datafiles to a Tablespace

• Changing a Datafile's Size

• Altering Datafile Availability

• Renaming and Relocating Datafiles

• Verifying Data Blocks in Datafiles

• Viewing Information About Datafiles

  See Also: Datafiles can also be created as part of database recovery from a media failure. For more information, see the Oracle8i Backup and Recovery Guide.

Guidelines for Managing Datafiles

This section describes aspects of managing datafiles, and includes the following topics:

• Determine the Number of Datafiles

• Set the Size of Datafiles

• Place Datafiles Appropriately

• Store Datafiles Separate From Redo Log Files

Every datafile has two associated file numbers: an absolute file number and a relative file number.

An absolute file number uniquely identifies a datafile in the database. In earlier releases of Oracle, the absolute file number may have been referred to as simply, the "file number."

A relative file number uniquely identifies a datafile within a tablespace. For small and medium size databases, relative file numbers usually have the same value as the absolute file number. However, when the number of datafiles in a database exceeds a threshold (typically 1023), the relative file number will differ from the absolute file number. You can locate relative file numbers in many data dictionary views.

Determine the Number of Datafiles

At least one datafile is required for the SYSTEM tablespace of a database; a small system might have a single datafile. In general, keeping a few large datafiles is preferable to many small datafiles, because you can keep fewer files open at the same time.

You can add datafiles to tablespaces, subject to the following operating system-specific datafile limits:

• Operating system limit

  Each operating system sets a limit on the maximum number of open files per process. Regardless of all other limits, more datafiles cannot be created when the operating system limit of open files is reached.

• Oracle system limit

  Oracle imposes a maximum limit on the number of datafiles for any Oracle database opened by any instance. This limit is port-specific.

• Control file upper bound

  When you issue CREATE DATABASE or CREATE CONTROLFILE statements, the MAXDATAFILES parameter specifies an initial size of the datafile portion of the control file. Later, if you add a file whose number exceeds MAXDATAFILES but is less than or equal to the value specified by the DB_FILES initialization parameter, the control file automatically expands to allow the datafile portion to accommodate more files.

• Instance or SGA upper bound

  When starting an Oracle8 instance, the database's initialization parameter file indicates the amount of SGA space to reserve for datafile information; the maximum number of datafiles is controlled by the DB_FILES initialization parameter. This limit applies only for the life of the instance.

  Note: The default value of DB_FILES is operating system specific.

  With the Oracle Parallel Server, all instances must set the instance datafile upper bound to the same value.

When determining a value for DB_FILES, take the following into consideration:

• If the value of DB_FILES is too low, you will be unable to add datafiles beyond the DB_FILES limit without first shutting down the database.

• If the value of DB_FILES is too high, memory is unnecessarily consumed.

Theoretically, an Oracle database can have an unlimited number of datafiles. Nevertheless, you should consider the following when determining the number of datafiles:

• Performance is better with a small number of datafiles rather than a large number of small datafiles. A large number of files also increases the granularity of a recoverable unit.

• Operating systems often impose a limit on the number of files a process can open simultaneously. Oracle's DBWn processes can open all online datafiles. Oracle is also capable of treating open file descriptors as a cache, automatically closing files when the number of open file descriptors reaches the operating system-defined limit.

Oracle allows more datafiles in the database than the operating system-defined limit; this can have a negative performance impact. When possible, adjust the operating system limit on open file descriptors so that it is larger than the number of online datafiles in the database.

The operating system specific limit on the maximum number of datafiles allowed in a tablespace is typically 1023 files.

See Also: For more information on operating system limits, see your operating system-specific Oracle documentation. For information about Parallel Server operating system limits, see Oracle8i Parallel Server Administration, Deployment, and Performance. For more information about MAXDATAFILES parameter of the CREATE DATABASE or CREATE CONTROLFILE statement, see the Oracle8i SQL Reference.

Set the Size of Datafiles

The first datafile (in the original SYSTEM tablespace) must be at least 7M to contain the initial data dictionary and rollback segment. If you install other Oracle products, they may require additional space in the SYSTEM tablespace (for online help, for example); see the installation instructions for these products.

Place Datafiles Appropriately

Tablespace location is determined by the physical location of the datafiles that constitute that tablespace. Use the hardware resources of your computer appropriately.

For example, if several disk drives are available to store the database, it might be helpful to store table data in a tablespace on one disk drive, and index data in a tablespace on another disk drive. This way, when users query table information, both disk drives can work simultaneously, retrieving table and index data at the same time.

Store Datafiles Separate From Redo Log Files

Datafiles should not be stored on the same disk drive that stores the database's redo log files. If the datafiles and redo log files are stored on the same disk drive and that disk drive fails, the files cannot be used in your database recovery procedures.

If you multiplex your redo log files, then the likelihood of losing all of your redo log files is low, so you can store datafiles on the same drive as some redo log files.

Creating and Adding Datafiles to a Tablespace

Ideally, when creating a tablespace, you should estimate the potential size of the database objects and add sufficient files or devices, so as to ensure that data is spread evenly across all devices. Later, if needed, you can create and add datafiles to a tablespace to increase the total amount of disk space allocated for the tablespace, and consequently the database.

To add datafiles to a tablespace, you use the ALTER TABLESPACE...ADD DATAFILE statement. You must have the ALTER TABLESPACE system privilege to add datafiles to a tablespace.

The following statement creates a new datafile for the RB_SEGS tablespace:

ALTER TABLESPACE rb_segs
   ADD DATAFILE '/u02/oracle/rbdb1/rb_segs03.dbf' SIZE 1M;

If you add new datafiles to a tablespace and do not fully specify the filenames, Oracle creates the datafiles in the default database directory or the current directory, depending upon your operating system. Oracle recommends you always specify a fully qualified name for a datafile. Unless you want to reuse existing files, make sure the new filenames do not conflict with other files. Old files that have been previously dropped will be overwritten.

Changing a Datafile's Size

This section describes the various ways to alter the size of a datafile, and includes the following topics:

• Enabling and Disabling Automatic Extension for a Datafile

• Manually Resizing a Datafile

Enabling and Disabling Automatic Extension for a Datafile

You can create datafiles or alter existing datafiles so that they automatically increase in size when more space is needed in the database. The files increase in specified increments up to a specified maximum.

Setting your datafiles to extend automatically results in the following:

• Reduces the need for immediate intervention when a tablespace runs out of space

• Ensures applications will not halt because of failures to allocate extents

To find out if a datafile is auto-extensible, query the DBA_DATA_FILES view and examine the AUTOEXTENSIBLE column.

You can specify automatic file extension by specifying an AUTOEXTEND ON clause when you create datafiles using the following SQL statements:

• CREATE DATABASE

• CREATE TABLESPACE

• ALTER TABLESPACE

You can enable or disable automatic file extension for existing datafiles, or manually resize a datafile using the SQL statement ALTER DATABASE.

The following example enables automatic extension for a datafile added to the USERS tablespace:

ALTER TABLESPACE users
    ADD DATAFILE '/u02/oracle/rbdb1/users03.dbf' SIZE 10M
      AUTOEXTEND ON
      NEXT 512K
      MAXSIZE 250M;

The value of NEXT is the minimum size of the increments added to the file when it extends. The value of MAXSIZE is the maximum size to which the file can automatically extend.

The next example disables the automatic extension for the datafile.

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/users03.dbf' 
    AUTOEXTEND OFF;

See Also: For more information about the SQL statements for creating or altering datafiles, see the Oracle8i SQL Reference.

Manually Resizing a Datafile

You can manually increase or decrease the size of a datafile using the ALTER DATABASE statement.

Because you can change the sizes of datafiles, you can add more space to your database without adding more datafiles. This is beneficial if you are concerned about reaching the maximum number of datafiles allowed in your database.

Manually reducing the sizes of datafiles allows you to reclaim unused space in the database. This is useful for correcting errors in estimates of space requirements.

In the next example, assume that the datafile /u02/oracle/rbdb1/stuff01.dbf has extended up to 250M. However, because its tablespace now stores smaller objects, the datafile can be reduced in size.

The following statement decreases the size of datafile /u02/oracle/rbdb1/stuff01.dbf:

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/stuff01.dbf'
   RESIZE 100M;

Note: It is not always possible to decrease the size of a file to a specific value.

See Also: For more information about the implications resizing files has for downgrading, see Oracle8i Migration

Altering Datafile Availability

This section describes ways to alter datafile availability, and includes the following topics:

• Bringing Datafiles Online or Taking Offline in ARCHIVELOG Mode

• Taking Datafiles Offline in NOARCHIVELOG Mode

In very rare situations, you might need to bring specific datafiles online (make them available) or take specific files offline (make them unavailable). For example, when Oracle has problems writing to a datafile, it can automatically take the datafile offline. You might need to take the damaged datafile offline or bring it online manually

Note: You can make all datafiles in a tablespace, other than the files in the SYSTEM tablespace, temporarily unavailable by taking the tablespace offline. You must leave these files in the tablespace to bring the tablespace back online. For more information about taking a tablespace offline, see "Taking Tablespaces Offline".

Offline datafiles cannot be accessed. Bringing online a datafile in a read-only tablespace makes the file readable. No one can write to the file unless its associated tablespace is returned to the read-write state. The files of a read-only tablespace can independently be taken online or offline using the DATAFILE option of the ALTER DATABASE statement.

To bring a datafile online or take it offline, you must have the ALTER DATABASE system privilege. You can perform these operations only when the database is open in exclusive mode.

Bringing Datafiles Online or Taking Offline in ARCHIVELOG Mode

To bring an individual datafile online, issue the ALTER DATABASE statement and include the DATAFILE clause.The following statement brings the specified datafile online:

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/stuff01.dbf' ONLINE;

To take the same file offline, issue the following statement:

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/stuff01.dbf' OFFLINE;

Note: To use this option of the ALTER DATABASE statement, the database must be in ARCHIVELOG mode. This requirement prevents you from accidentally losing the datafile, since taking the datafile offline while in NOARCHIVELOG mode is likely to result in losing the file.

See Also: For more information about bringing datafiles online during media recovery, see the Oracle8i Backup and Recovery Guide.

Taking Datafiles Offline in NOARCHIVELOG Mode

To take a datafile offline when the database is in NOARCHIVELOG mode, use the ALTER DATABASE statement with both the DATAFILE and OFFLINE DROP clauses. This allows you to take the datafile offline and drop it immediately. It is useful, for example, if the datafile contains only data from temporary segments and has not been backed up and the database is in NOARCHIVELOG mode.

The following statement takes the specified datafile offline:

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/users03.dbf' OFFLINE DROP;

Renaming and Relocating Datafiles

You can rename datafiles to either change their names or relocate them. Some options, and procedures which you can follow, are described in the following sections:

• Renaming and Relocating Datafiles for a Single Tablespace

  For example, renaming filename1 and filename2 in tablespace1, while the rest of the database is open.

• Renaming and Relocating Datafiles for Multiple Tablespaces

  For example, renaming filename1 in tablespace1 and filename2 in tablespace2, while the database is mounted but closed.

  Note: To rename or relocate datafiles of the SYSTEM tablespace, you must use the second option, because you cannot take the SYSTEM tablespace offline.

When you rename and relocate datafiles with these procedures, only the pointers to the datafiles, as recorded in the database's control file, are changed; they do not physically rename any operating system files, nor do they copy files at the operating system level. Therefore, renaming and relocating datafiles involves several steps. Read the steps and examples carefully before performing these procedures.

Renaming and Relocating Datafiles for a Single Tablespace

These are some procedures for renaming and relocating datafiles in a single tablespace. You must have the ALTER TABLESPACE system privilege to rename datafiles of a single tablespace.

Renaming Datafiles in a Single Tablespace

To rename datafiles from a single tablespace, follow this procedure.

1. Take the non-SYSTEM tablespace that contains the datafiles offline.

2. Rename the datafiles using operating system statements.

3. Make sure that the new, fully specified filenames are different from the old filenames.

4. Use the ALTER TABLESPACE statement with the RENAME DATAFILE option to change the filenames within the database.

For example, the following statement renames the datafiles filename1 and filename2 to filename3 and filename4, respectively:

ALTER TABLESPACE users
    RENAME DATAFILE '/u02/oracle/rbdb1/user1.dbf',
                    '/u02/oracle/rbdb1/user2.dbf'
                 TO '/u02/oracle/rbdb1/users01.dbf', 
                    '/u02/oracle/rbdb1/users02.dbf';

The new files must already exist; this statement does not create the files. Also, always provide complete filenames (including their paths) to properly identify the old and new datafiles. In particular, specify the old datafile name exactly as it appears in the DBA_DATA_FILES view of the data dictionary.

Relocating and Renaming Datafiles in a Single Tablespace

Here is an example that illustrates the steps involved for relocating a datafile.

Assume the following conditions:

• An open database has a tablespace named USERS that is made up of datafiles located on the same disk of a computer.

• The datafiles of the USERS tablespace are to be relocated to a different disk drives.

• You are currently connected with administrator privileges to the open database.

These are the steps:

1. Identify the datafile names of interest.

   The following query of the data dictionary view DBA_DATA_FILES lists the datafile names and respective sizes (in bytes) of the USERS tablespace:

   SELECT file_name, bytes FROM sys.dba_data_files
   WHERE tablespace_name = 'USERS';
   
   FILE_NAME                                  BYTES
   ------------------------------------------ ----------------
   /U02/ORACLE/RBDB1/USERS01.DBF              102400000
   /U02/ORACLE/RBDB1/USERS02.DBF              102400000
   

2. Back up the database.

   Before making any structural changes to a database, such as renaming and relocating the datafiles of one or more tablespaces, always completely back up the database.

3. Take the tablespace containing the datafiles offline, or shut down the database and restart and mount it, leaving it closed. Either option closes the datafiles of the tablespace.

4. Copy the datafiles to their new locations and rename them using operating system commands.

   Note: You can execute an operating system command to copy a file by using the HOST command.

5. Rename the datafiles within Oracle.

   The datafile pointers for the files that make up the USERS tablespace, recorded in the control file of the associated database, must now be changed from the old names to the new names.

   If the tablespace is offline but the database is open, use the ALTER TABLESPACE...RENAME DATAFILE statement. If the database is mounted but closed, use the ALTER DATABASE...RENAME FILE statement.

   ALTER TABLESPACE users
       RENAME DATAFILE '/u02/oracle/rbdb1/users01.dbf',
                       '/u02/oracle/rbdb1/users02.dbf'
                    TO '/u03/oracle/rbdb1/users01.dbf', 
                       '/u04/oracle/rbdb1/users02.dbf';
   
   

6. Bring the tablespace online, or shut down and restart the database.

   If the USERS tablespace is offline and the database is open, bring the tablespace back online. If the database is mounted but closed, open the database.

7. Back up the database. After making any structural changes to a database, always perform an immediate and complete backup.

Renaming and Relocating Datafiles for Multiple Tablespaces

You can rename and relocate datafiles of one or more tablespaces using ALTER DATABASE statement with the RENAME FILE option. This option is the only choice if you want to rename or relocate datafiles of several tablespaces in one operation, or rename or relocate datafiles of the SYSTEM tablespace. If the database must remain open, consider instead the procedure outlined in the previous section.

To rename datafiles of several tablespaces in one operation or to rename datafiles of the SYSTEM tablespace, you must have the ALTER DATABASE system privilege.

To rename datafiles in multiple tablespaces, follow these steps.

1. Ensure that the database is mounted but closed.

2. Copy the datafiles to be renamed to their new locations and new names, using operating system commands.

3. Make sure the new copies of the datafiles have different fully specified filenames from the datafiles currently in use.

4. Use ALTER DATABASE to rename the file pointers in the database's control file.

For example, the following statement renames the datafiles filename1 and filename2 to filename3 and filename4, respectively:

ALTER DATABASE
    RENAME FILE '/u02/oracle/rbdb1/sort01.dbf',
                '/u02/oracle/rbdb1/user3.dbf'
             TO '/u02/oracle/rbdb1/temp01.dbf',
                '/u02/oracle/rbdb1/users03.dbf;

The new file must already exist; this statement does not create a file. Also, always provide complete filenames (including their paths) to properly identify the old and new datafiles. In particular, specify the old datafile name exactly as it appears in the DBA_DATA_FILES view of the data dictionary.

Verifying Data Blocks in Datafiles

If you want to configure Oracle to use checksums to verify data blocks, set the initialization parameter DB_BLOCK_CHECKSUM to TRUE. The value of this parameter can be changed dynamically, or set in the initialization parameter file. The default value of DB_BLOCK_CHECKSUM is FALSE. Regardless of the setting of this parameter, checksums will always be used to verify data blocks in the system tablespace.

When you enable block checking, Oracle computes a checksum for each block written to disk. Checksums are computed for all data blocks, including temporary blocks.

The DBWn process calculates the checksum for each block and stores it in the block's header. Checksums are also computed by the direct loader.

The next time Oracle reads a data block, it uses the checksum to detect corruption in the block. If a corruption is detected, Oracle returns message ORA-01578 and writes information about the corruption to a trace file.

WARNING: Setting DB_BLOCK_CHECKSUM to TRUE can cause performance overhead. Set this parameter to TRUE only under the advice of Oracle Support personnel to diagnose data corruption problems.

Viewing Information About Datafiles

The following data dictionary views provide useful information about the datafiles of a database:

View	Description
DBA_DATA_FILES	Provides descriptive information about datafiles, including the tablespace to which it belong and the file id. The file id can be used to join with other views for detail information.
USER_EXTENTS, DBA_EXTENTS	Lists the extents comprising all segments in the database. Contains the file id of the datafile containing the extent.
USER_FREE_SPACE, DBA_FREE_SPACE	Lists the free extents in all tablespaces. Includes the file id of the datafile containing the extent.
V$DATAFILE	Contains datafile information from the control file.
V$DATAFILE_HEADER	Contains information from datafile headers.

This example illustrates the use of one of these views, V$DATAFILE.

Assume you are using a database that contains two tablespaces, SYSTEM and USERS. USERS is made up of two files, FILE1 (100MB) and FILE2 (200MB); the tablespace has been taken offline normally. Here, you query V$DATAFILE to view status information about datafiles of a database:

SELECT name,
    file#,
    status,
    checkpoint_change# "CHECKPOINT"   
  FROM   v$datafile;

NAME                                      FILE#     STATUS       CHECKPOINT
--------------------------------          -----     -------       ----------
filename1                                     1     SYSTEM              3839
filename2                                     2     OFFLINE             3782
filename3                                     3     OFFLINE             3782

FILE# lists the file number of each datafile; the first datafile in the SYSTEM tablespace created with the database is always file 1. STATUS lists other information about a datafile. If a datafile is part of the SYSTEM tablespace, its status is SYSTEM (unless it requires recovery). If a datafile in a non-SYSTEM tablespace is online, its status is ONLINE. If a datafile in a non-SYSTEM tablespace is offline, its status can be either OFFLINE or RECOVER. CHECKPOINT lists the final SCN written for a datafile's most recent checkpoint.

See Also: For a complete description of these views, see Oracle8i Reference.