A single file creation form is used to create a single file, such as a data file or a control file. The form of the Oracle ASM file name in this type is either an alias, such as +data/orcl/ctl.f, or an incomplete file name, such as a disk group name preceded by a plus sign (+data). You use the alias or disk group name where a file name is called for in a SQL statement, such as CREATE TABLESPACE, CREATE CONTROLFILE.

A multiple file creation form is used multiple times to create Oracle ASM files, such as when assigning a value to the initialization parameter DB_CREATE_FILE_DEST. You can also run a CREATE TABLESPACE statement (without a file name specification) multiple times. Each time, Oracle ASM creates a different unique data file name.

When using the multiple file creation form, you can use an incomplete file name form, such as a disk group name preceded by a plus sign (+data). In this case, you set DB_CREATE_FILE_DEST to an incomplete file name (for example, +data), and whenever a command is run that must create a database file in DB_CREATE_FILE_DEST, the file is created in the designated disk group and assigned a unique fully qualified name. You can use an incomplete file name in other *_DEST initialization parameters.

You can use a fully qualified form for referencing existing Oracle ASM files in Oracle ASM operations, except for disk group creation. This form is the Oracle ASM file name that Oracle ASM automatically generates when an Oracle ASM file is created.

A fully qualified file name has the following form in a non-multitenant environment:

+diskgroup/dbname/filetype/filetypetag.file.incarnation

With multitenant databases, a fully qualified file name has the following form:

+diskgroup/cdb/pdb_guid/filetype/filetypetag.file.incarnation

The fields of the fully qualified file name are:

• +diskgroup is the disk group name preceded by a plus sign.

  You can think of the plus sign (+) as the root directory of the Oracle ASM file system, similar to the slash (/) on UNIX or Linux computers.

• dbname is the DB_UNIQUE_NAME of the database to which the file belongs.

• filetype is the Oracle file type and can be one of the file types shown in Table 5-3.

• filetypetag is type-specific information about the file and can be one of the tags shown in Table 5-3.

• file.incarnation is the file/incarnation pair, used to ensure uniqueness.

• cdb is the name of the CDB

• pdb_guid is the PDB GUID value.

Examples of fully qualified Oracle ASM file names are:

SQL> SELECT NAME FROM V$DATAFILE; 

NAME
---------------------------------------------------------------------------
+DATA/ORCL/DATAFILE/system.257.967544915
+DATA/ORCL/DATAFILE/sysaux.258.967544961
+DATA/ORCL/DATAFILE/undotbs1.259.967544985
+DATA/ORCL/64A52F53A7693286E053CDA9E80AED76/DATAFILE/system.267.967545119
+DATA/ORCL/64A52F53A7693286E053CDA9E80AED76/DATAFILE/sysaux.266.967545119
+DATA/ORCL/DATAFILE/users.260.967544987
...

You can use Alias Oracle ASM file names both for referencing existing Oracle ASM files and for creating new Oracle ASM files. Alias names start with the disk group name preceded by a plus sign, after which you specify a name string. Alias file names are implemented using a hierarchical directory structure, with the slash (/) or backslash (\) character separating name components. You can create an alias in any Oracle ASM directory generated by Oracle Managed Files (system-generated) or created by a user. An alias must include the disk group name; you cannot create an alias at the root level (+).

When you create an Oracle ASM file with an alias file name, the file is created with a fully qualified name, and the alias file name is additionally created. You can then access the file with either name.

Alias Oracle ASM file names are distinguished from fully qualified file names or numeric file names because they do not end in a dotted pair of numbers. It is an error to attempt to create an alias that ends in a dotted pair of numbers, such as in the format USERS.259.685366091. Examples of Oracle ASM alias file names are:

+data/orcl/control_file1
+fra/recover/second.dbf

Oracle Database references database files by their alias file names, but only if you create the database files with aliases. If you create database files without aliases and then add aliases later, the database references the files by their fully qualified file names. The following are examples of how the database uses alias file names:

• Alias file names appear in V$ views. For example, if you create a tablespace and use an alias file name for the data file, the V$DATAFILE view shows the alias file name.

• When a control file points to data files and online redo log files, it can use alias file names.

• The CONTROL_FILES initialization parameter can use the alias file names of the control files. Database Configuration Assistant (DBCA) creates control files with alias file names.

For more information about alias file names, see Managing Alias Names for Oracle ASM File Names.

Example 5-1 creates an undo tablespace with a data file that has an alias name, and with attributes that are set by the user-defined template my_undo_template. This example assumes that a directory has been created in disk group data to contain the alias name and that the user-defined template exists.

Because an alias is used when creating the data file, the file is not an Oracle Managed Files (OMF) file and the file is not automatically deleted when the tablespace is dropped. To drop the file manually after the tablespace has been dropped, use the following SQL statement:

ALTER DISKGROUP data DROP FILE '+data/orcl/my_undo_ts';

CREATE UNDO TABLESPACE myundo 
     DATAFILE '+data(my_undo_template)/orcl/my_undo_ts' SIZE 200M; 

An alias Oracle ASM file name with template is used only for Oracle ASM file creation operations. It has the following format:

+diskgroup(templatename)/alias

Alias file names with template behave identically to alias file names except that a file created with an alias file name with template receives the attributes specified by the named template. The template must belong to the disk group that the file is being created in.

An example of an alias Oracle ASM file name with template is:

+data(mytemplate)/config1

Explicitly specifying a template name, as in the previous example, overrides the system default template for the type of file being created.

Incomplete Oracle ASM file names are used only for file creation operations and are used for both single and multiple file creation. They consist only of the disk group name. Oracle ASM uses a system default template to determine the Oracle ASM file mirroring and striping attributes. The system template that is used is determined by the file type that is being created. For example, if you are creating a data file for a tablespace, then the DATAFILE template is used.

An example of using an incomplete Oracle ASM file name is setting the DB_CREATE_FILE_DEST initialization parameter to:

+data

With this setting, every time you create a tablespace, a data file is created in the disk group data, and each data file is assigned a different fully qualified name. See Creating Oracle ASM Files Using a Default File Location for Disk Group Specification for more information.

Incomplete Oracle ASM file names with templates are used only for file creation operations and are used for both single and multiple file creation. They consist of the disk group name followed by the template name in parentheses. When you explicitly specify a template in a file name, Oracle ASM uses the specified template instead of the default template for that file type to determine attributes for the file.

An example of using an incomplete Oracle ASM file name with template is setting the DB_CREATE_FILE_DEST initialization parameter to:

+data(mytemplate)

Using the Oracle Managed Files feature for operating system files, you can specify a directory as the default location for the creation of data files, temporary files, redo log files, and control files. Using the Oracle Managed Files feature for Oracle ASM, you can specify a disk group, in the form of an incomplete Oracle ASM file name, as the default location for creation of these files, and additional types of files, including archived log files. As for operating system files, the name of the default disk group is stored in an initialization parameter and is used whenever a file specification (for example, DATAFILE clause) is not explicitly specified during file creation.

The following initialization parameters accept the multiple file creation context form of Oracle ASM file names as a destination:

• DB_CREATE_FILE_DEST

  Specifies the default disk group location in which to create data and temp files.

• DB_CREATE_ONLINE_LOG_DEST_n

  Specifies the default disk group location in which to create redo log and control files.

• DB_RECOVERY_FILE_DEST

  Specifies a default disk group for a fast recovery area that contains a copy of the control file and redo log files if this parameter is specified and DB_CREATE_ONLINE_LOG_DEST_n and CONTROL_FILES are not specified.

• CONTROL_FILES

  Specifies a disk group in which to create control files.

The following initialization parameters accept the multiple file creation context form of the Oracle ASM file names and Oracle ASM directory names as a destination:

• LOG_ARCHIVE_DEST_n

  Specifies a default disk group or Oracle ASM directory as destination for archiving redo log files

• LOG_ARCHIVE_DEST

  Optional parameter to use to specify a default disk group or Oracle ASM directory as destination for archiving redo log files. Use when specifying only one destination.

• STANDBY_ARCHIVE_DEST

  Relevant only for a standby database. It specifies a default disk group or Oracle ASM directory that is the location of archive logs arriving from a primary database. Not discussed in this book. This parameter has been deprecated. See Oracle Data Guard Concepts and Administration.

The following CREATE TABLESPACE SQL statement illustrates how an Oracle ASM file, in this case a data file, might be created in a default disk group. This example assumes that the DB_CREATE_FILE_DEST initialization parameter has been set to +data.

SQL> CREATE TABLESPACE mytblspace;

Oracle ASM automatically creates and manages the data file for mytblspace on Oracle ASM disks in the disk group data. File extents are stored using the attributes defined by the default template for a data file.

You can specify Oracle ASM file names in the file specification clause of your SQL statements. If you are creating a file for the first time, then use the creation form of an Oracle ASM file name. If the Oracle ASM file exists, then you must use the reference form of the file name, and if you are trying to re-create the file, you must add the REUSE keyword so that the space is reused for the new file. This usage might occur when, for example, trying to re-create a control file, as shown in Creating Control Files in Oracle ASM.

If a reference form is used with the REUSE keyword and the file does not exist, an error results.

Example 5-2 is an example of specifying an Oracle ASM file name in a SQL statement. In this case, it is used in the file creation form.

The tablespace mytblspace is created and comprises one data file of size 200 MB contained in the disk group data. The data file is set to auto-extensible with an unlimited maximum size. You can use an AUTOEXTEND clause to override this default.

For more examples of the use of Oracle ASM file name in SQL statements, see Creating Database Files in Oracle ASM Using SQL*Plus.

CREATE TABLESPACE mytblspace DATAFILE '+data' SIZE 200M AUTOEXTEND ON;

You can use the ALTER DATABASE MOVE DATAFILE SQL statement to move data files between disk groups while the database is open and users are accessing the data files.

Example 5-3 shows how you can move a data file from the DATA disk group to the DATA2 disk group.

/* Create a new disk group DATA2 using ASMCA, ASMCMD, or SQL */
/* Then create appropriate directories in the DATA2 disk group */

ALTER DISKGROUP data2 ADD DIRECTORY '+DATA2/ORCL';

ALTER DISKGROUP data2 ADD DIRECTORY '+DATA2/ORCL/DATAFILE';

/* Move the EXAMPLE data file in DATA to EXAMPLE_STORAGE in DATA2
ALTER DATABASE MOVE DATAFILE '+DATA/ORCL/DATAFILE/EXAMPLE.266.798707687' 
      TO '+DATA2/ORCL/DATAFILE/EXAMPLE_STORAGE';

SELECT SUBSTR(name,1,64) FROM V$DATAFILE;
 
SUBSTR(NAME,1,64)
----------------------------------------------------------------
+DATA/ORCL/DATAFILE/system.258.798707555
+DATA2/ORCL/DATAFILE/example_storage
+DATA/ORCL/DATAFILE/sysaux.257.798707491
+DATA/ORCL/DATAFILE/undotbs1.260.798707621
+DATA/ORCL/DATAFILE/users.259.798707621

For an understanding of I/O performance, you must have detailed knowledge of the storage hierarchy in which files reside.

Oracle Database provides a set of dynamic performance views to show a complete mapping of a file to intermediate layers of logical volumes to actual physical devices. Using these dynamic views, you can locate the exact disk on which any block of a file resides.

Oracle Database communicates with a background process called FMON to manage the mapping information. Oracle provides the PL/SQL DBMS_STORAGE_MAP package to invoke mapping operations that populate the mapping views.

Oracle Database file mapping does not require third party libraries when mapping Oracle ASM files. In addition, Oracle Database supports Oracle ASM file mapping on all operating system platforms.

All the examples in this section are run on the Oracle Database instance.

This section contains the following topics:

• Enabling File Mapping With Oracle ASM Files

• Using File Mapping Views With Oracle ASM Files

• Using the DBMS_STORAGE_MAP Package With Oracle ASM Files

Use the ADD ALIAS clause of the ALTER DISKGROUP statement to create an alias name for an Oracle ASM file name. The alias name must consist of the full directory path and the alias itself.

Example 5-4 adds a new alias name for a system-generated file name.

ALTER DISKGROUP data ADD ALIAS '+data/orcl/second.dbf'
     FOR '+data/orcl/datafile/mytable.342.123456789';

Use the RENAME ALIAS clause of the ALTER DISKGROUP statement to rename an alias for an Oracle ASM file name. The old and the new alias names must consist of the full directory paths of the alias names.

Example 5-5 renames an alias.

Renaming an alias does not update the database reference to the alias.

ALTER DISKGROUP data RENAME ALIAS '+data/orcl/datafile.dbf'
     TO '+data/payroll/compensation.dbf';

Use the DROP ALIAS clause of the ALTER DISKGROUP statement to drop an alias for an Oracle ASM file name. The alias name must consist of the full directory path and the alias itself. The underlying file to which the alias refers is unchanged.

Example 5-6 drops an alias.

Example 5-7 fails because it attempts to drop a system-generated file name and this syntax is not allowed.

ALTER DISKGROUP data DROP ALIAS '+data/payroll/compensation.dbf';

ALTER DISKGROUP data 
     DROP ALIAS '+data/orcl/datafile/mytable.342.123456789';

You can delete Oracle ASM files and their associated aliases from a disk group using the DROP FILE clause of the ALTER DISKGROUP statement. You must use a fully qualified file name, or an alias name when specifying the file to delete. The underlying file on the file system is not dropped when the alias is dropped.

Some reasons why you may want to delete files include:

• A file created using aliases is not an Oracle Managed File. Consequently, it is not automatically deleted.

• A point-in-time-recovery of a database might restore the database to a time before a tablespace was created. The restore does not delete the tablespace, but there is no reference to the tablespace or its data file in the restored database. You could manually delete the data file.

In Example 5-8, the alias name for the file is used when deleting both the file and the alias from a disk group.

In Example 5-9, the Oracle Managed Files (system-generated) file name is used when dropping the file and any associated alias.

ALTER DISKGROUP data DROP FILE '+data/payroll/compensation.dbf';

ALTER DISKGROUP data
     DROP FILE '+data/orcl/datafile/mytable.342.123456789';

Use the ADD DIRECTORY clause of the ALTER DISKGROUP statement to create a hierarchical directory structure for alias names for Oracle ASM files. Use the slash (/) or backslash (\) character to separate components of the directory path. The directory path must start with the disk group name, preceded by a plus sign (+), followed by any subdirectory names of your choice.

The parent directory must exist before attempting to create a subdirectory or alias in that directory. A directory must be created below the disk group level.

Example 5-10 creates a hierarchical directory for disk group data, which can contain, for example, the alias name +data/orcl/control_file1.

Assuming no subdirectory exists under the directory +data/orcl, the SQL statement fails in Example 5-11.

ALTER DISKGROUP data ADD DIRECTORY '+data/orcl';

ALTER DISKGROUP data
     ADD DIRECTORY '+data/orcl/first_dir/second_dir';

The RENAME DIRECTORY clause of the ALTER DISKGROUP statement enables you to rename a directory. System-created directories (those containing system-generated names) cannot be renamed. The root level (disk group name) cannot be renamed.

Example 5-12 renames a directory.

ALTER DISKGROUP data RENAME DIRECTORY '+data/mydir'
     TO '+data/yourdir';

You can delete a directory using the DROP DIRECTORY clause of the ALTER DISKGROUP statement. You cannot drop a system-created directory. You cannot drop a directory containing alias names unless you also specify the FORCE clause.

Example 5-13 deletes a directory along with its contents.

ALTER DISKGROUP data DROP DIRECTORY '+data/yourdir' FORCE;

The Oracle ASM virtual folder is created by default during XML DB installation. If the database is not configured to use Oracle ASM, the folder is empty and no operations are permitted on it.

The Oracle ASM virtual folder contains folders and subfolders that follow the hierarchy defined by the structure of an Oracle ASM fully qualified file name. Figure 5-1 illustrates an example of this hierarchy, which for simplicity, excludes directories created for aliases.

The folder /sys/asm contains one subfolder for every mounted disk group, and each disk group folder contains one subfolder for each database that uses the disk group. In addition, a disk group folder might contain files and folders corresponding to aliases created by the administrator. Continuing the hierarchy, the database folders contain file type folders, which contain the Oracle ASM files.

Figure 5-1 Hierarchical Structure of Oracle ASM Folders in an XML DB Installation

Description of "Figure 5-1 Hierarchical Structure of Oracle ASM Folders in an XML DB Installation"

The following are usage restrictions on /sys/asm:

• You cannot create hard links to existing Oracle ASM files or directories with APIs such as DBMS_XDB.LINK.

• You cannot rename (move) an Oracle ASM file to another disk group or to a directory outside Oracle ASM.

You can use the directory /sys/asm for storing the names of disk groups. You cannot store other files in this directory. Within the disk group directories under /sys/asm, such as /sys/asm/DATA, you can only store database files in these sub-directories. Oracle ASM rejects attempts to store non-database files in these directories.

Table 5-4 shows the permitted striping attribute values and allocation unit size chunks. These values correspond to the STRIPE column of V$ASM_TEMPLATE.

Table 5-5 shows the permitted redundancy values for Oracle ASM templates. These values correspond to the REDUND column of V$ASM_TEMPLATE.

Table 5-6 shows the initial attribute settings for some default templates. The type of mirroring associated with the Mirroring column for normal, high, and external redundancy disk groups is specified in Table 5-5. For example, the Mirror column for high redundancy disk groups displays MIRROR in Table 5-6. In Table 5-5, the MIRROR value for high redundancy disk groups corresponds to Three-way mirroring.

Table 5-7 shows the permitted primary and mirror region values for Oracle ASM templates. These values correspond to the PRIMARY_REGION and MIRROR_REGION columns of V$ASM_TEMPLATE.

To add a template to a disk group, use the ADD TEMPLATE clause of the ALTER DISKGROUP statement.

If the name of your new template is not listed in Table 5-6, then it is not used as a default template for database file types. To use the template, you must reference its name when creating a file.

When adding a template to a disk group, the attributes are optional. If no redundancy attribute is specified, then the value defaults to MIRROR for a normal redundancy disk group, HIGH for a high redundancy disk group, and UNPROTECTED for an external redundancy disk group. If you do not specify a striping attribute, then the value defaults to COARSE. If you do not specify a value for the primary or mirror region attributes, then the value defaults to COLD and MIRRORCOLD respectively.

Example 5-14 creates a new template named reliable for the normal redundancy disk group data.

Example 5-15 creates a new template named unreliable that specifies UNPROTECTED (no mirroring) for files.

ALTER DISKGROUP data ADD TEMPLATE reliable ATTRIBUTES (HIGH FINE);

ALTER DISKGROUP data ADD TEMPLATE unreliable ATTRIBUTES (UNPROTECTED);

The MODIFY TEMPLATE clause of the ALTER DISKGROUP statement enables you to modify the attribute specifications of an existing system default or user-defined disk group template. Only specified template attributes are changed. Unspecified properties retain their current value. When you modify an existing template, only new files created by the template reflect the attribute changes. Existing files maintain their attributes.

Example 5-16 changes the striping attribute specification of the reliable template for disk group data.

ALTER DISKGROUP data MODIFY TEMPLATE reliable 
     ATTRIBUTES (COARSE);

Use the DROP TEMPLATE clause of the ALTER DISKGROUP statement to drop one or more templates from a disk group. You can only drop templates that are user-defined; you cannot drop system default templates.

Example 5-17 drops the previously created template unreliable from data:

ALTER DISKGROUP data DROP TEMPLATE unreliable;

Use the SQL ALTER SYSTEM and CREATE TABLESPACE statements to create a tablespace that uses a user-defined template to specify the attributes of the data file.

Example 5-18 assumes that the template (mytemplate) has been defined.

ALTER SYSTEM SET DB_CREATE_FILE_DEST = '+data(mytemplate)';

CREATE TABLESPACE mytblspace;