Oracle Automatic Storage Management (Oracle ASM) is a volume manager and a file system for Oracle database files that supports single-instance Oracle Database and Oracle Real Application Clusters (Oracle RAC) configurations.

Oracle ASM uses disk groups to store data files; an Oracle ASM disk group is a collection of disks that Oracle ASM manages as a unit. Within a disk group, Oracle ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group is evenly distributed to eliminate hot spots and to provide uniform performance across the disks. The performance is comparable to the performance of raw devices.

You can add or remove disks from a disk group while a database continues to access files from the disk group. When you add or remove disks from a disk group, Oracle ASM automatically redistributes the file contents and eliminates the need for downtime when redistributing the content.

The Oracle ASM volume manager functionality provides flexible server-based mirroring options. The Oracle ASM normal and high redundancy disk groups enable two-way and three-way mirroring respectively. You can use external redundancy to enable a Redundant Array of Independent Disks (RAID) storage subsystem to perform the mirroring protection function.

Oracle ASM also uses the Oracle Managed Files feature to simplify database file management. Oracle Managed Files automatically creates files in designated locations. Oracle Managed Files also names files and removes them while relinquishing space when tablespaces or files are deleted.

Oracle ASM reduces the administrative overhead for managing database storage by consolidating data storage into a small number of disk groups. The smaller number of disk groups consolidates the storage for multiple databases and provides for improved I/O performance.

Oracle ASM files can coexist with other storage management options such as raw disks and third-party file systems. This capability simplifies the integration of Oracle ASM into pre-existing environments.

Oracle ASM has easy to use management interfaces such as SQL*Plus, the Oracle ASM Command Line Utility (ASMCMD) command-line interface, and Oracle ASM Configuration Assistant (ASMCA).

All data files (including an undo tablespace for each instance) and redo log files (at least two for each instance) for an Oracle RAC database must reside on shared storage.

Oracle recommends that you use Oracle ASM to store these files in an Oracle ASM disk group.

Oracle supports alternative ways of using shared storage, such as certified cluster file systems. In addition, Oracle recommends that you use one shared server parameter file (SPFILE) with instance-specific entries. Oracle RAC allows storing shared password files in Oracle ASM and storing Oracle Database files on Oracle Automatic Storage Management Cluster File System (Oracle ACFS).

Unless otherwise noted, Oracle Database storage features such as Oracle ASM, Oracle Managed Files, automatic segment-space management, and so on, function the same in Oracle RAC environments as they do in non-cluster Oracle database environments.

All Oracle RAC instances must be able to access all data files. If a data file must be recovered when the database is opened, then the first Oracle RAC instance to start is the instance that performs the recovery and verifies access to the file. As other instances start, they also verify their access to the data files. Similarly, when you add a tablespace or data file or bring a tablespace or data file online, all instances verify access to the file or files.

If you add a data file to a disk that other instances cannot access, then verification fails. Verification also fails if instances access different copies of the same data file. If verification fails for any instance, then diagnose and fix the problem. Then run the ALTER SYSTEM CHECK DATAFILES statement on each instance to verify data file access.

An Oracle database can serve as a network file system (NFS) server. The database responds to NFS requests from any NFS client and stores both the files and their metadata within the database.

Files associated with a primary database, such as SQL scripts, can be automatically replicated on a standby database. You can also store unstructured data, such as emails, on the database.

You can create or destroy an Oracle file system and access it though the NFS server using the procedures documented in Oracle Database SecureFiles and Large Objects Developer's Guide.

Learn about redo log requirements for Oracle Real Application Clusters (Oracle RAC) databases.

In an Oracle RAC database, each instance must have at least two groups of redo log files. When you use DBCA to create the database, DBCA allocates redo log files to instances, as required, automatically. You can change the number of redo log groups and the size of the redo log files as required either during the initial database creation or as a post-creation step. If you add a node to the cluster, then the addNode script configures the redo logs on the new server.

When the current group fills, an instance begins writing to the next log file group. If your database is in ARCHIVELOG mode, then each instance must save filled online log groups as archived redo log files that are tracked in the control file. During database recovery, all enabled instances are checked to see if recovery is needed. If you remove an instance from your Oracle RAC database, then you should disable the instance's thread of redo so that Oracle does not have to check the thread during database recovery.

Oracle Database automatically manages undo segments within a specific undo tablespace that is assigned to an instance.

Instances can always read all undo blocks throughout the cluster environment for consistent read purposes. Also, any instance can update any undo tablespace during transaction recovery, if that undo tablespace is not currently used by another instance for undo generation or transaction recovery.

You assign undo tablespaces in your Oracle Real Application Clusters (Oracle RAC) database by specifying a different value for the UNDO_TABLESPACE parameter for each instance in your SPFILE or individual PFILEs. Oracle automatically allocates the undo tablespace when the instance starts if you have Oracle Managed Files enabled. You cannot simultaneously use automatic undo management and manual undo management in an Oracle RAC database. In other words, all instances of an Oracle RAC database must operate in the same undo mode.

Oracle Automatic Storage Management (Oracle ASM) automatically maximizes I/O performance by managing the storage configuration across the disks that Oracle ASM manages.

Oracle ASM does this by evenly distributing the database files across all of the available storage assigned to the disk groups within Oracle ASM. Oracle ASM allocates your total disk space requirements into uniformly sized units across all disks in a disk group. Oracle ASM can also automatically mirror files to prevent data loss. Because of these features, Oracle ASM also significantly reduces your administrative overhead.

Oracle ASM instances are created on each node where you install Oracle Clusterware. Each Oracle ASM instance has either an SPFILE or PFILE type parameter file. Oracle recommends that you back up the parameter files and the TNS entries for nondefault Oracle Net listeners.

To use Oracle ASM with Oracle Real Application Clusters (Oracle RAC), select Oracle ASM as your storage option when you create your database with the Oracle Database Configuration Assistant (DBCA). As in noncluster Oracle databases, using Oracle ASM with Oracle RAC does not require I/O tuning.

• Storage Management in Oracle RAC
  You can create Oracle ASM disk groups and configure mirroring for Oracle ASM disk groups using the Oracle ASM configuration assistant (ASMCA).
• Modifying Disk Group Configurations for Oracle ASM
  When you create a disk group for a cluster or add new disks to an existing clustered disk group, prepare the underlying physical storage on shared disks and give the Oracle user permission to read and write to the disk.
• Oracle ASM Disk Group Management
  To use Oracle ASM, you must first create disk groups with ASMCA before creating a database with DBCA.
• Configuring Preferred Mirror Read Disks in Extended Distance Clusters
  You can configure preferred read disks to improve performance.
• Converting Nonclustered Oracle ASM to Clustered Oracle ASM
  
• Administering Oracle ASM Instances with SRVCTL in Oracle RAC
  You can use the Server Control Utility (SRVCTL) to add or remove an Oracle ASM instance.