About Oracle Automatic Storage Management

Oracle Automatic Storage Management (Oracle ASM) is an integrated, high-performance database file system and disk manager. You use Oracle ASM instead of an operating system file system to store your Oracle Database files and, optionally, other files.

Oracle ASM is based on the principle that the database should manage storage instead of requiring an administrator to do it. Oracle ASM eliminates the need for you to manage potentially thousands of database files.

Oracle ASM groups the disks in your storage system into one or more disk groups. You manage a small set of disk groups, and Oracle ASM automates the placement of the database files within those disk groups.

Oracle ASM provides the following benefits:

• Striping—Oracle ASM spreads data evenly across all disks in a disk group to optimize performance and utilization. This even distribution of database files eliminates the need for regular monitoring and I/O performance tuning.

  For example, if there are six disks in a disk group, then pieces of each Oracle ASM file are written to all six disks. These pieces are known as extents. In this scenario, these pieces come in 1 MB chunks. When a database file is created, it is striped (divided into extents and distributed) across the six disks, and allocated disk space on all six disks increases evenly. When reading the file, file extents are read from all six disks in parallel, greatly increasing performance.

• Mirroring—Oracle ASM can increase availability by optionally mirroring any file. Oracle ASM mirrors at the file level, unlike operating system mirroring, which mirrors at the disk level. Mirroring means keeping redundant copies, or mirrored copies, of each extent of the file to help avoid data loss caused by disk failures. The mirrored copy of each file extent is always kept on a different disk from the original copy. If a disk fails, then Oracle ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.

  Oracle ASM supports 2-way mirroring, where each file extent gets one mirrored copy, and 3-way mirroring, where each file extent gets two mirrored copies.

• Online storage reconsideration and dynamic rebalancing—Oracle ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, Oracle ASM automatically redistributes the data so that it is evenly spread across all disks in the disk group, including the new disk. This redistribution is known as rebalancing. It is done in the background and with minimal impact to database performance. When you request to remove a disk, Oracle ASM first rebalances the disk group by evenly relocating all file extents from the disk being removed to the other disks in the disk group.

• Managed file creation and deletion—Oracle ASM further reduces administrative tasks by enabling files stored in Oracle ASM disk groups to be Oracle-managed files. Oracle ASM automatically assigns file names when files are created, and automatically deletes files when they are no longer needed. For information about Oracle-managed files, see Oracle Database Administrator's Guide.

Oracle recommends that you use Oracle ASM for your database file storage, instead of raw devices or the operating system file system. However, databases can have a mixture of Oracle ASM files and non-Oracle ASM files. Oracle Enterprise Manager includes a wizard that enables you to migrate non-Oracle ASM database files to Oracle ASM.

The Oracle ASM Instance

Oracle ASM is implemented as a special kind of Oracle instance, with its own System Global Area and background processes. The Oracle ASM instance is tightly integrated with the database instance. Every server running one or more database instances that use Oracle ASM for storage has an Oracle ASM instance. In an Oracle Real Application Clusters environment, there is one Oracle ASM instance for each node, and the Oracle ASM instances communicate with each other on a peer-to-peer basis. Only one Oracle ASM instance is required for each node, regardless of the number of database instances on the node.

Administering Oracle ASM

You administer Oracle ASM with Oracle Enterprise Manager Database Control (Database Control). To administer the Oracle ASM instance and Oracle ASM disk groups, you must connect to the Oracle ASM instance as a user who has been granted the SYSASM system privilege. You can also administer Oracle ASM with Oracle Automatic Storage Management Command Line utility (ASMCMD) and SQL*Plus statements.

When you create the Oracle ASM instance either during installation or using Oracle Automatic Storage Management Configuration Assistant (ASMCA), the SYSASM is granted to the specified Oracle ASM administrator user.