Automatic Storage Management
Encyclopedia
Automatic Storage Management (ASM) is a feature provided by Oracle Corporation
within the Oracle Database
from release Oracle 10g
(revision 1) onwards. ASM aims to simplify the management of database
files. To do so, it provides tools to manage file system
s and volume
s directly inside the database, allowing database administrator
s (DBAs) to control volumes and disks with familiar SQL
statements in standard Oracle environments. Thus DBAs do not need extra skills in specific file system
s or volume managers
(which usually operate at the level of the operating system
).
With ASM:
s. The system then spreads these extents across all disks in a "diskgroup". One can think of a diskgroup in ASM as an LVM
volume group
— with an ASM file corresponding to a logical volume. In addition to the existing Oracle background processes, ASM introduces two new ones - OSMB and RBAL. RBAL opens and creates disks in a diskgroup. OSMB provides the functionality of moving data between disks in a diskgroup.
The ASM functionality is controlled by an ASM instance. This is not a full database instance, just the memory structures, and as such is very small and lightweight.
The main components of ASM are disk groups, each of which comprise several physical disks controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are known as ASM files. The locations and names for the files are controlled by ASM, but user-friendly aliases and directory structures can be defined for ease of reference.
The level of redundancy and the granularity of the striping can be controlled using templates. Oracle Corporation provides default templates for each file-type stored by ASM, but additional templates can be defined as needed.
Failure groups are defined within a disk group to support the required level of redundancy. For two-way mirroring a disk group might contain two failure groups, in which case individual files are written to two locations.
In summary ASM provides the following functionality:
Incorrect usage of parameters in ASM or RDBMS instances results in ORA-15021 errors.
To create an ASM instance first create a file called init+ASM.ora in the /tmp directory containing the following information.
INSTANCE_TYPE=ASM
Next, using SQL*Plus, connect to the ide instance.
export ORACLE_SID=+ASM
sqlplus / as sysdba
Create an spfile using the contents of the init+ASM.ora file.
SQL> CREATE SPFILE FROM PFILE='/tmp/init+ASM.ora';
File created.
Finally, start the instance with the NOMOUNT option.
SQL> startup nomount
ASM instance started
Total System Global Area 125829120 bytes
Fixed Size 1301456 bytes
Variable Size 124527664 bytes
Database Buffers 0 bytes
Redo Buffers 0 bytes
SQL>
The ASM instance is now ready to use for creating and mounting disk groups. To shut down the ASM instance, issue the following command:
SQL> shutdown
ASM instance shutdown
SQL>
Once an ASM instance is present, disk groups can be used for the following parameters in database instances (INSTANCE_TYPE=RDBMS) to allow ASM file creation:
* DB_CREATE_FILE_DEST
* DB_CREATE_ONLINE_LOG_DEST_n
* DB_RECOVERY_FILE_DEST
* CONTROL_FILES
* LOG_ARCHIVE_DEST_n
* LOG_ARCHIVE_DEST
* STANDBY_ARCHIVE_DEST
* FORCE - Performs a SHUTDOWN ABORT before restarting the ASM instance.
* MOUNT - Starts the ASM instance and mounts the disk groups specified by the ASM_DISKGROUPS parameter.
* NOMOUNT - Starts the ASM instance without mounting any disk groups.
* OPEN - This is not a valid option for an ASM instance.
The options for the SHUTDOWN command are:
* NORMAL - The ASM instance waits for all connected ASM instances and SQL sessions to exit then shuts down.
* IMMEDIATE - The ASM instance waits for any SQL transactions to complete then shuts down. It doesn't wait for sessions to exit.
* TRANSACTIONAL - Same as IMMEDIATE.
* ABORT - The ASM instance shuts down instantly.
* NORMAL REDUNDANCY - Two-way mirroring, requiring two failure groups.
* HIGH REDUNDANCY - Three-way mirroring, requiring three failure groups.
* EXTERNAL REDUNDANCY - No mirroring for disks that are already protected using hardware mirroring or RAID.
In addition, failure groups and preferred names for disks can be defined. If the NAME clause is omitted the disks are given a system generated name like "disk_group_1_0001". The FORCE option can be used to move a disk from another disk group into this one.
CREATE DISKGROUP disk_group_1 NORMAL REDUNDANCY
FAILGROUP failure_group_1 DISK
'/devices/diska1' NAME diska1,
'/devices/diska2' NAME diska2,
FAILGROUP failure_group_2 DISK
'/devices/diskb1' NAME diskb1,
'/devices/diskb2' NAME diskb2;
Disk groups can be deleted using the DROP DISKGROUP statement.
DROP DISKGROUP disk_group_1 INCLUDING CONTENTS;
Disks can be added or removed from disk groups using the ALTER DISKGROUP statement. Remember that the wildcard "*" can be used to reference disks so long as the resulting string does not match a disk already used by an existing disk group.
-- Add disks.
ALTER DISKGROUP disk_group_1 ADD DISK
'/devices/disk*3',
'/devices/disk*4';
-- Drop a disk.
ALTER DISKGROUP disk_group_1 DROP DISK diska2;
Disks can be resized using the RESIZE clause of the ALTER DISKGROUP statement. The statement can be used to resize individual disks, all disks in a failure group or all disks in the disk group. If the SIZE clause is omitted the disks are resized to the size of the disk returned by the OS.
-- Resize a specific disk.
ALTER DISKGROUP disk_group_1
RESIZE DISK diska1 SIZE 100G;
-- Resize all disks in a failure group.
ALTER DISKGROUP disk_group_1
RESIZE DISKS IN FAILGROUP failure_group_1 SIZE 100G;
-- Resize all disks in a disk group.
ALTER DISKGROUP disk_group_1
RESIZE ALL SIZE 100G;
The UNDROP DISKS clause of the ALTER DISKGROUP statement allows pending disk drops to be undone. It will not revert drops that have completed, or disk drops associated with the dropping of a disk group.
ALTER DISKGROUP disk_group_1 UNDROP DISKS;
Disk groups can be rebalanced manually using the REBALANCE clause of the ALTER DISKGROUP statement. If the POWER clause is omitted the ASM_POWER_LIMIT parameter value is used. Rebalancing is only needed when the speed of the automatic rebalancing is not appropriate.
ALTER DISKGROUP disk_group_1 REBALANCE POWER 5;
Disk groups are mounted at ASM instance startup and unmounted at ASM instance shutdown. Manual mounting and dismounting can be accomplished using the ALTER DISKGROUP statement as seen below.
ALTER DISKGROUP ALL DISMOUNT;
ALTER DISKGROUP ALL MOUNT;
ALTER DISKGROUP disk_group_1 DISMOUNT;
ALTER DISKGROUP disk_group_1 MOUNT;
-- Create a new template.
ALTER DISKGROUP disk_group_1 ADD TEMPLATE my_template ATTRIBUTES (MIRROR FINE);
-- Modify template.
ALTER DISKGROUP disk_group_1 ALTER TEMPLATE my_template ATTRIBUTES (COARSE);
-- Drop template.
ALTER DISKGROUP disk_group_1 DROP TEMPLATE my_template;
Available attributes include:
-- Create a directory.
ALTER DISKGROUP disk_group_1 ADD DIRECTORY '+disk_group_1/my_dir';
-- Rename a directory.
ALTER DISKGROUP disk_group_1 RENAME DIRECTORY '+disk_group_1/my_dir' TO '+disk_group_1/my_dir_2';
-- Delete a directory and all its contents.
ALTER DISKGROUP disk_group_1 DROP DIRECTORY '+disk_group_1/my_dir_2' FORCE;
-- Create an alias using the fully qualified filename.
ALTER DISKGROUP disk_group_1 ADD ALIAS '+disk_group_1/my_dir/my_file.dbf'
FOR '+disk_group_1/mydb/datafile/my_ts.342.3';
-- Create an alias using the numeric form filename.
ALTER DISKGROUP disk_group_1 ADD ALIAS '+disk_group_1/my_dir/my_file.dbf'
FOR '+disk_group_1.342.3';
-- Rename an alias.
ALTER DISKGROUP disk_group_1 RENAME ALIAS '+disk_group_1/my_dir/my_file.dbf'
TO '+disk_group_1/my_dir/my_file2.dbf';
-- Delete an alias.
ALTER DISKGROUP disk_group_1 DELETE ALIAS '+disk_group_1/my_dir/my_file.dbf';
Attempting to drop a system alias results in an error.
-- Drop file using an alias.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1/my_dir/my_file.dbf';
-- Drop file using a numeric form filename.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1.342.3';
-- Drop file using a fully qualified filename.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1/mydb/datafile/my_ts.342.3';
-- Check metadata for a specific file.
ALTER DISKGROUP disk_group_1 CHECK FILE '+disk_group_1/my_dir/my_file.dbf'
-- Check metadata for a specific failure group in the disk group.
ALTER DISKGROUP disk_group_1 CHECK FAILGROUP failure_group_1;
-- Check metadata for a specific disk in the disk group:
ALTER DISKGROUP disk_group_1 CHECK DISK diska1; -- Check metadata for all disks in the disk group. ALTER DISKGROUP disk_group_1 CHECK ALL;
Views include:
CREATE TABLESPACE my_ts DATAFILE '+disk_group_1' SIZE 100M AUTOEXTEND ON;
* Disable change tracking (only available in Enterprise Edition) if it is currently being used.
SQL> ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;
* Shutdown the database.
SQL> SHUTDOWN IMMEDIATE
* Modify the parameter file of the target database as follows:
o Set the DB_CREATE_FILE_DEST and DB_CREATE_ONLINE_LOG_DEST_n parameters to the relevant ASM disk groups.
o Remove the CONTROL_FILES parameter from the spfile so the control files will be moved to
the DB_CREATE_* destination and the spfile gets updated automatically. If you are using
a pfile the CONTROL_FILES parameter must be set to the appropriate ASM files or aliases.
* Start the database in nomount mode.
RMAN> STARTUP NOMOUNT
* Restore the controlfile into the new location from the old location.
RMAN> RESTORE CONTROLFILE FROM 'old_control_file_name';
* Mount the database.
RMAN> ALTER DATABASE MOUNT;
* Copy the database into the ASM disk group.
RMAN> BACKUP AS COPY DATABASE FORMAT '+disk_group';
* Switch all datafile to the new ASM location.
RMAN> SWITCH DATABASE TO COPY;
* Open the database.
RMAN> ALTER DATABASE OPEN;
* Create new redo logs in ASM and delete the old ones.
* Enable change tracking if it was being used.
SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING;
ASM stripes help make data more reliably available and more secure than in other Oracle storage implementations.
+///file
Note that the root starts with '+' — for example:
+EXTERNAL/ORCL/DATA/system.54.1234567
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
within the Oracle Database
Oracle Database
The Oracle Database is an object-relational database management system produced and marketed by Oracle Corporation....
from release Oracle 10g
Oracle Database
The Oracle Database is an object-relational database management system produced and marketed by Oracle Corporation....
(revision 1) onwards. ASM aims to simplify the management of database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
files. To do so, it provides tools to manage file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
s and volume
Volume (computing)
In the context of computer operating systems, volume is the term used to describe a single accessible storage area with a single file system, typically resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on...
s directly inside the database, allowing database administrator
Database administrator
A database administrator is a person responsible for the design, implementation, maintenance and repair of an organization's database. They are also known by the titles Database Coordinator or Database Programmer, and is closely related to the Database Analyst, Database Modeller, Programmer...
s (DBAs) to control volumes and disks with familiar SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
statements in standard Oracle environments. Thus DBAs do not need extra skills in specific file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
s or volume managers
Logical volume management
In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes...
(which usually operate at the level of the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
).
With ASM:
- IOInput/outputIn computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
channels can take advantage of data stripingData stripingIn computer data storage, data striping is the technique of segmenting logically sequential data, such as a file, in a way that accesses of sequential segments are made to different physical storage devices. Striping is useful when a processing device requests access to data more quickly than a...
and software mirroringMirror (computing)In computing, a mirror is an exact copy of a data set. On the Internet, a mirror site is an exact copy of another Internet site.Mirror sites are most commonly used to provide multiple sources of the same information, and are of particular value as a way of providing reliable access to large downloads... - DBAs can automate online redistribution of data, along with the addition and removal of disks/storage
- the system maintains redundant copies and provides 3rd-party RAIDRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
functionality - Oracle supports third-party multipathing IO technologies (such as failoverFailoverIn computing, failover is automatic switching to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active application, server, system, or network...
or load balancingLoad balancing (computing)Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid...
to SANStorage area networkA storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
access) - the need for hot spareHot spareA hot spare or hot standby is used as a failover mechanism to provide reliability in system configurations. The hot spare is active and connected as part of a working system. When a key component fails, the hot spare is switched into operation...
s diminishes
Features
- ASMCMD: useful for users not familiar with SQL (one can write scripts with it)
Architecture overview
ASM creates extents out of datafiles, log-files, system files, control files and other database structureDatabase storage structures
Database tables/indexes are typically stored on hard disk in one of many forms, ordered/unordered Flat files, ISAM, Heaps, Hash buckets or B+ Trees. These have various advantages and disadvantages discussed in this topic. The most commonly used are B+trees and ISAM....
s. The system then spreads these extents across all disks in a "diskgroup". One can think of a diskgroup in ASM as an LVM
Logical Volume Manager
Logical Volume Manager may refer to:*Logical Volume Manager *Logical Volume Manager...
volume group
Volume group
Volume Group is the highest level abstraction used within the Logical Volume Manager . It gathers together a collection of Logical Volumes and Physical Volumes into one administrative unit....
— with an ASM file corresponding to a logical volume. In addition to the existing Oracle background processes, ASM introduces two new ones - OSMB and RBAL. RBAL opens and creates disks in a diskgroup. OSMB provides the functionality of moving data between disks in a diskgroup.
Implementation and usage
Automatic Storage Management (ASM), introduced in Oracle 10g. aims to simplify the storage of Oracle datafiles, controlfiles and logfiles.Overview
Automatic Storage Management (ASM) simplifies administration of Oracle-related files by allowing the administrator to reference disk groups (rather than individual disks and files) which ASM manages. The ASM functionality is an extension of the Oracle Managed Files (OMF) functionality that also includes striping and mirroring to provide balanced and secure storage. The new ASM functionality can be used in combination with existing raw and cooked file systems, along with OMF and manually-managed files.The ASM functionality is controlled by an ASM instance. This is not a full database instance, just the memory structures, and as such is very small and lightweight.
The main components of ASM are disk groups, each of which comprise several physical disks controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are known as ASM files. The locations and names for the files are controlled by ASM, but user-friendly aliases and directory structures can be defined for ease of reference.
The level of redundancy and the granularity of the striping can be controlled using templates. Oracle Corporation provides default templates for each file-type stored by ASM, but additional templates can be defined as needed.
Failure groups are defined within a disk group to support the required level of redundancy. For two-way mirroring a disk group might contain two failure groups, in which case individual files are written to two locations.
In summary ASM provides the following functionality:
- Manages groups of disks, called disk groups.
- Manages disk redundancy within a disk group.
- Provides near-optimal I/O balancing without any manual tuning.
- Enables management of database objects without specifying mount-points and filenames.
- Supports large files.
Setup
The following initialization parameters relate specifically to an ASM instance:- INSTANCE_TYPE - set to ASM or RDBMS depending on the instance type. The default is RDBMS.
- DB_UNIQUE_NAME - specifies a globally unique name for the database. This defaults to +ASM but must be altered if one intends to run multiple ASM instances.
- ASM_POWER_LIMIT - the maximum power for a rebalancing operation on an ASM instance. The valid values range from 1 to 11, with 1 being the default. The higher the limit the more resources are allocated resulting in faster rebalancing operations. This value is also used as the default when the POWER clause is omitted from a rebalance operation.
- ASM_DISKGROUPS - the list of disk groups that should be mounted by an ASM instance during instance startup, or by the ALTER DISKGROUP ALL MOUNT statement. ASM configuration changes are automatically reflected in this parameter.
- ASM_DISKSTRING - specifies a value that can be used to limit the disks considered for discovery. Altering the default value may improve the speed of disk group mount time and the speed of adding a disk to a disk group. Changing the parameter to a value which prevents the discovery of already mounted disks results in an error. The default value is NULL allowing all suitable disks to be considered.
Incorrect usage of parameters in ASM or RDBMS instances results in ORA-15021 errors.
To create an ASM instance first create a file called init+ASM.ora in the /tmp directory containing the following information.
INSTANCE_TYPE=ASM
Next, using SQL*Plus, connect to the ide instance.
export ORACLE_SID=+ASM
sqlplus / as sysdba
Create an spfile using the contents of the init+ASM.ora file.
SQL> CREATE SPFILE FROM PFILE='/tmp/init+ASM.ora';
File created.
Finally, start the instance with the NOMOUNT option.
SQL> startup nomount
ASM instance started
Total System Global Area 125829120 bytes
Fixed Size 1301456 bytes
Variable Size 124527664 bytes
Database Buffers 0 bytes
Redo Buffers 0 bytes
SQL>
The ASM instance is now ready to use for creating and mounting disk groups. To shut down the ASM instance, issue the following command:
SQL> shutdown
ASM instance shutdown
SQL>
Once an ASM instance is present, disk groups can be used for the following parameters in database instances (INSTANCE_TYPE=RDBMS) to allow ASM file creation:
* DB_CREATE_FILE_DEST
* DB_CREATE_ONLINE_LOG_DEST_n
* DB_RECOVERY_FILE_DEST
* CONTROL_FILES
* LOG_ARCHIVE_DEST_n
* LOG_ARCHIVE_DEST
* STANDBY_ARCHIVE_DEST
Startup and shutdown of ASM instances
ASM instance are started and stopped in a similar way to normal database instances. The options for the STARTUP command are:* FORCE - Performs a SHUTDOWN ABORT before restarting the ASM instance.
* MOUNT - Starts the ASM instance and mounts the disk groups specified by the ASM_DISKGROUPS parameter.
* NOMOUNT - Starts the ASM instance without mounting any disk groups.
* OPEN - This is not a valid option for an ASM instance.
The options for the SHUTDOWN command are:
* NORMAL - The ASM instance waits for all connected ASM instances and SQL sessions to exit then shuts down.
* IMMEDIATE - The ASM instance waits for any SQL transactions to complete then shuts down. It doesn't wait for sessions to exit.
* TRANSACTIONAL - Same as IMMEDIATE.
* ABORT - The ASM instance shuts down instantly.
Disks
Disk groups are created using the CREATE DISKGROUP statement. This statement allows specification of the level of redundancy:* NORMAL REDUNDANCY - Two-way mirroring, requiring two failure groups.
* HIGH REDUNDANCY - Three-way mirroring, requiring three failure groups.
* EXTERNAL REDUNDANCY - No mirroring for disks that are already protected using hardware mirroring or RAID.
In addition, failure groups and preferred names for disks can be defined. If the NAME clause is omitted the disks are given a system generated name like "disk_group_1_0001". The FORCE option can be used to move a disk from another disk group into this one.
CREATE DISKGROUP disk_group_1 NORMAL REDUNDANCY
FAILGROUP failure_group_1 DISK
'/devices/diska1' NAME diska1,
'/devices/diska2' NAME diska2,
FAILGROUP failure_group_2 DISK
'/devices/diskb1' NAME diskb1,
'/devices/diskb2' NAME diskb2;
Disk groups can be deleted using the DROP DISKGROUP statement.
DROP DISKGROUP disk_group_1 INCLUDING CONTENTS;
Disks can be added or removed from disk groups using the ALTER DISKGROUP statement. Remember that the wildcard "*" can be used to reference disks so long as the resulting string does not match a disk already used by an existing disk group.
-- Add disks.
ALTER DISKGROUP disk_group_1 ADD DISK
'/devices/disk*3',
'/devices/disk*4';
-- Drop a disk.
ALTER DISKGROUP disk_group_1 DROP DISK diska2;
Disks can be resized using the RESIZE clause of the ALTER DISKGROUP statement. The statement can be used to resize individual disks, all disks in a failure group or all disks in the disk group. If the SIZE clause is omitted the disks are resized to the size of the disk returned by the OS.
-- Resize a specific disk.
ALTER DISKGROUP disk_group_1
RESIZE DISK diska1 SIZE 100G;
-- Resize all disks in a failure group.
ALTER DISKGROUP disk_group_1
RESIZE DISKS IN FAILGROUP failure_group_1 SIZE 100G;
-- Resize all disks in a disk group.
ALTER DISKGROUP disk_group_1
RESIZE ALL SIZE 100G;
The UNDROP DISKS clause of the ALTER DISKGROUP statement allows pending disk drops to be undone. It will not revert drops that have completed, or disk drops associated with the dropping of a disk group.
ALTER DISKGROUP disk_group_1 UNDROP DISKS;
Disk groups can be rebalanced manually using the REBALANCE clause of the ALTER DISKGROUP statement. If the POWER clause is omitted the ASM_POWER_LIMIT parameter value is used. Rebalancing is only needed when the speed of the automatic rebalancing is not appropriate.
ALTER DISKGROUP disk_group_1 REBALANCE POWER 5;
Disk groups are mounted at ASM instance startup and unmounted at ASM instance shutdown. Manual mounting and dismounting can be accomplished using the ALTER DISKGROUP statement as seen below.
ALTER DISKGROUP ALL DISMOUNT;
ALTER DISKGROUP ALL MOUNT;
ALTER DISKGROUP disk_group_1 DISMOUNT;
ALTER DISKGROUP disk_group_1 MOUNT;
Templates
Templates are named groups of attributes that can be applied to the files within a disk group. The following example show how templates can be created, altered and dropped.-- Create a new template.
ALTER DISKGROUP disk_group_1 ADD TEMPLATE my_template ATTRIBUTES (MIRROR FINE);
-- Modify template.
ALTER DISKGROUP disk_group_1 ALTER TEMPLATE my_template ATTRIBUTES (COARSE);
-- Drop template.
ALTER DISKGROUP disk_group_1 DROP TEMPLATE my_template;
Available attributes include:
- UNPROTECTED - No mirroring or striping regardless of the redundancy setting.
- MIRROR - Two-way mirroring for normal redundancy and three-way mirroring for high redundancy. This attribute cannot be set for external redundancy.
- COARSE - Specifies lower granuality for striping. This attribute cannot be set for external redundancy.
- FINE - Specifies higher granularity for striping. This attribute cannot be set for external redundancy.
Directories
A directory hierarchy can be defined using the ALTER DISKGROUP statement to support ASM file aliasing. The following examples show how ASM directories can be created, modified and deleted.-- Create a directory.
ALTER DISKGROUP disk_group_1 ADD DIRECTORY '+disk_group_1/my_dir';
-- Rename a directory.
ALTER DISKGROUP disk_group_1 RENAME DIRECTORY '+disk_group_1/my_dir' TO '+disk_group_1/my_dir_2';
-- Delete a directory and all its contents.
ALTER DISKGROUP disk_group_1 DROP DIRECTORY '+disk_group_1/my_dir_2' FORCE;
Aliases
Aliases allow the referencing of ASM files using user-friendly names, rather than the fully qualified ASM filenames.-- Create an alias using the fully qualified filename.
ALTER DISKGROUP disk_group_1 ADD ALIAS '+disk_group_1/my_dir/my_file.dbf'
FOR '+disk_group_1/mydb/datafile/my_ts.342.3';
-- Create an alias using the numeric form filename.
ALTER DISKGROUP disk_group_1 ADD ALIAS '+disk_group_1/my_dir/my_file.dbf'
FOR '+disk_group_1.342.3';
-- Rename an alias.
ALTER DISKGROUP disk_group_1 RENAME ALIAS '+disk_group_1/my_dir/my_file.dbf'
TO '+disk_group_1/my_dir/my_file2.dbf';
-- Delete an alias.
ALTER DISKGROUP disk_group_1 DELETE ALIAS '+disk_group_1/my_dir/my_file.dbf';
Attempting to drop a system alias results in an error.
Files
Files are not deleted automatically if they are created using aliases, as they are not Oracle Managed Files (OMF), or if a recovery is done to a point-in-time before the file was created. For these circumstances it is necessary to manually delete the files, as shown below.-- Drop file using an alias.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1/my_dir/my_file.dbf';
-- Drop file using a numeric form filename.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1.342.3';
-- Drop file using a fully qualified filename.
ALTER DISKGROUP disk_group_1 DROP FILE '+disk_group_1/mydb/datafile/my_ts.342.3';
Checking metadata
The internal consistency of disk group metadata can be checked in a number of ways using the CHECK clause of the ALTER DISKGROUP statement.-- Check metadata for a specific file.
ALTER DISKGROUP disk_group_1 CHECK FILE '+disk_group_1/my_dir/my_file.dbf'
-- Check metadata for a specific failure group in the disk group.
ALTER DISKGROUP disk_group_1 CHECK FAILGROUP failure_group_1;
-- Check metadata for a specific disk in the disk group:
ALTER DISKGROUP disk_group_1 CHECK DISK diska1; -- Check metadata for all disks in the disk group. ALTER DISKGROUP disk_group_1 CHECK ALL;
ASM views
The ASM configuration can be viewed using the V$ASM_% views, which often contain different information depending on whether they are queried from the ASM instance, or a dependant database instance.Views include:
- V$ASM_ALIAS Displays a row for each alias present in every disk group mounted by the ASM instance. Returns no rows
- V$ASM_CLIENT Displays a row for each database instance using a disk group managed by the ASM instance. Displays a row for the ASM instance if the database has open ASM files.
- V$ASM_DISK Displays a row for each disk discovered by the ASM instance, including disks which are not part of any disk group. Displays a row for each disk in disk groups in use by the database instance.
- V$ASM_DISKGROUP Displays a row for each disk group discovered by the ASM instance. Displays a row for each disk group mounted by the local ASM instance.
- V$ASM_FILE Displays a row for each file for each disk group mounted by the ASM instance. Displays no rows.
- V$ASM_OPERATION Displays a row for each file for each long running operation executing in the ASM instance. Displays no rows.
- V$ASM_TEMPLATE Displays a row for each template present in each disk group mounted by the ASM instance. Displays a row for each template present in each disk group mounted by the ASM instance with which the database instance communicates.
ASM Filenames
There are several ways to reference ASM file. Some forms are used during creation and some for referencing ASM files. The forms for file creation are incomplete, relying on ASM to create the fully qualified name, which can be retrieved from the supporting views. The forms of the ASM filenames are summarised below.- Fully Qualified ASM Filename +dgroup/dbname/file_type/file_type_tag.file.incarnation
- Numeric ASM Filename +dgroup.file.incarnation
- Alias ASM Filenames +dgroup/directory/filename
- Alias ASM Filename with Template +dgroup(template)/alias
- Incomplete ASM Filename +dgroup
- Incomplete ASM Filename with Template +dgroup(template)
SQL and ASM
ASM filenames can be used in place of conventional filenames for most Oracle file types, including controlfiles, datafiles, logfiles etc. For example, the following command creates a new tablespace with a datafile in the disk_group_1 disk group.CREATE TABLESPACE my_ts DATAFILE '+disk_group_1' SIZE 100M AUTOEXTEND ON;
Migrating to ASM Using RMAN
The following method shows how a primary database can be migrated to ASM from a disk based backup:* Disable change tracking (only available in Enterprise Edition) if it is currently being used.
SQL> ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;
* Shutdown the database.
SQL> SHUTDOWN IMMEDIATE
* Modify the parameter file of the target database as follows:
o Set the DB_CREATE_FILE_DEST and DB_CREATE_ONLINE_LOG_DEST_n parameters to the relevant ASM disk groups.
o Remove the CONTROL_FILES parameter from the spfile so the control files will be moved to
the DB_CREATE_* destination and the spfile gets updated automatically. If you are using
a pfile the CONTROL_FILES parameter must be set to the appropriate ASM files or aliases.
* Start the database in nomount mode.
RMAN> STARTUP NOMOUNT
* Restore the controlfile into the new location from the old location.
RMAN> RESTORE CONTROLFILE FROM 'old_control_file_name';
* Mount the database.
RMAN> ALTER DATABASE MOUNT;
* Copy the database into the ASM disk group.
RMAN> BACKUP AS COPY DATABASE FORMAT '+disk_group';
* Switch all datafile to the new ASM location.
RMAN> SWITCH DATABASE TO COPY;
* Open the database.
RMAN> ALTER DATABASE OPEN;
* Create new redo logs in ASM and delete the old ones.
* Enable change tracking if it was being used.
SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING;
Functionality
ASM requires an ASM instance to manage diskgroups. DBAs can use SQL*Plus to connect to ASM and to do the usual database-level operations like startup and shutdown. However, one must set the ORACLE_SID variable to '+ASM' and not to the ORACLE_SID of a database. Views like V$ASM_DISK, V$ASM_DISKGROUP, V$ASM_FILE, V$ASM_ALIAS make it easier to perform queries regarding disk groups, disks in each disk group, their layout, extents etc.Striping
ASM stripes its files across all the disks that belong to a disk group. It remains unclear if it follows a strict RAID 3 fashion of striping or a variant of RAID 3 that facilitates easy addition and removal of disks to and from the disk group. Oracle Corporation recommends that all the disks that belong to a disk group have the same size, in which case each disk gets the same number of extents. However, if a DBA configures disks of different sizes, each disk might get a different number of extents — based upon the size of the disk. An allocation unit typically has a size of 1MB.ASM stripes help make data more reliably available and more secure than in other Oracle storage implementations.
Redundancy
One can configure ASM diskgroups to have no redundancy (external), two-way mirroring (normal), or three-way mirroring (high). In the case of normal and high mirrors, good practice suggests having fail groups that talk to different controllers for performance and fail-safe reasons.- In the case of external redundancy, ASM does not do any software mirroring, but only stripes its files across all the disks that belong to the disk group that does external redundancy.
- In the case of normal redundancyRedundancy (engineering)In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
, ASM does two-way mirroring, meaning that ASM maintains two copies of the data through software mirroring. When querying for mirror information, DBAs will see two mirrors in this case. - In the case of high redundancy, ASM does three-way mirroring, maintaining three copies of the data through software mirroring. When querying for mirror information, DBAs will see three mirrors in this case.
Directory Structure
A typical directory of an ASM file looks like this:+
Note that the root starts with '+' — for example:
+EXTERNAL/ORCL/DATA/system.54.1234567
Comparison with other LVMs
| Other LVM | ASM |
|---|---|
| Volume Group | Disk Group |
| Volume | ASM File |
Advantages for database administrators
- Database administrators see a familiar interface: SQL*Plus
- Interaction with ASM happens through commands that closely resemble standard SQL
- DBAs can assign data, log, control and other Oracle database files to a specific disk group without bothering about the physical location of the disks that constitute a disk group
- If a disk group runs out of space, an additional disk can be added to the disk group without bothering to copy/move data and without bothering where exactly data lives on the physical disks.
External links
- http://twiki.cern.ch/twiki/pub/PSSGroup/Presentations2007/Inside_Oracle_ASM_LC_CERN_UKOUG07.ppt
- http://www.oracle.com/technetwork/issue-archive/2006/06-mar/o26asm-090635.html