About Automatic Storage Management

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

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

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

ASM provides the following benefits:

Striping—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, pieces of each ASM file are written to all six disks. These pieces come in 1 MB chunks known as extents. 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—ASM can increase availability by optionally mirroring any file. 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, ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.

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 reconfiguration and dynamic rebalancing—ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, 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, 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—ASM further reduces administrative tasks by enabling files stored in ASM disk groups to be Oracle-managed files. 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 ASM for your database file storage, instead of raw devices or the operating system file system. However, databases can have a mixture of ASM files and non-ASM files. Oracle Enterprise Manager includes a wizard that enables you to migrate non-ASM database files to ASM.

The ASM Instance

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

Administering ASM

You administer ASM with Oracle Enterprise Manager Database Control (Database Control). To administer the ASM instance and ASM disk groups, you must connect to the ASM instance as a user who has been granted the SYSASM system privilege. When you create the ASM instance with Database Configuration Assistant (DBCA), DBCA grants SYSASM to user SYS.