Oracle Automated Storage Management ASM Configuration

The Database Configuration Assistant (DBCA) is used in Oracle Database 10g to configure the Automated Storage Management (ASM) features for conventional and Real Application Clusters (RAC) environments. ASM is used to automate and simplify the optimal layout of datafiles, control files, and log files. ASM automatically distributes database files across all available disks, and whenever the storage configuration changes, the database storage is rebalanced. ASM can also be used to provide redundancy, through the mirroring of database files.

ASM essentially eliminates the management overhead involved with the use of a conventional file system. There is no need to manually place data on disks when using ASM. Also, by allowing the administrator to manage just a few disk groups, the administrator is relieved from managing hundreds or thousands of files. Using ASM, the DBA will use disk groups for setting up a database. After setup, the DBA need only be concerned with disk groups when monitoring and changing disk allocations within the disk groups.

ASM uses the process of breaking each file into multiple extents and spreading the extents evenly across all of the disks in a disk group. Once ASM disk groups are established, the Oracle database automatically allocates storage space from these disk groups for creating and deleting files.

Unneeded data files are automatically deleted with ASM, rather than requiring a manually issued command, as in previous versions. Automated Storage Management enhances database integrity for databases operating on disks that are not extremely reliable.

ASM doesn't have to be installed in order to install an Oracle database. However, to use ASM files, there must be at least one ASM instance configured and started prior to starting a database instance that uses ASM files.

Automated Storage Management ASM Disk Groups

A disk group is basically one or more ASM disks that are managed as a single logical unit. Any data-structure stored in an ASM disk group is totally contained within that disk group, or self-contained. A database using ASM disks doesn't have to be shutdown in order for a disk to be added or dropped. ASM rebalances the spread of data to ensure an even I/O load to all disks in a disk group when the disk group configuration changes.

We mentioned that any single ASM file is self-contained in a single ASM disk group. However, an ASM disk group can contain files belonging to several databases, and a single database can use storage from multiple ASM disk groups. One or more disk groups can be specified as the default disk group for files created in a database.

Disk groups are created at database creation or when a new application is developed. An ASM disk group can also change when its database server configuration is altered.

Most installations will probably have two or more disk groups. The reasons for having multiple different disk groups include the following:

§ To group disks of different manufacturers, different sizes or performance characteristics.

§ To group disks with different external redundancy together; for example, JBOD (just a bunch of disks) would generally not be in the same disk group with disks from a RAID 1+0 or RAID5 configuration, but this is possible using ASM.

§ To separate work and recovery areas for a given database.

In any installation, non-ASM managed operating system storage repositories are required, and are used for swap files, execution libraries, and user file systems. The Oracle database and ASM executable files and libraries must reside on the server’s operating system file system and cannot reside in an ASM disk.

There are three types of ASM disk groups:

- Normal redundancy
- High redundancy
- External redundancy

With normal and high redundancy, the disk group template specifies the ASM redundancy attributes for all files in the disk group. Configuration of ASM high redundancy provides a greater degree of protection. With external redundancy, ASM does not provide any redundancy for the disk group. In external redundancy, the underlying disks in the disk group must provide redundancy (for example, using a RAID storage array.)

ASM Files

As the database requests, ASM will create the required files. ASM assigns each file a fully qualified name ending in a dotted pair of numbers. You can create more user-friendly alias names for the Automated Storage Management filenames by using ASM aliases.

Query the v$osm_alias data dictionary view to see assigned alias names for ASM files. During normal operations, users do not need to be aware of ASM file names. The contents of the v$osm_alias view are shown in Table 1.1.

Whenever a filename is required from an ASM disk, a mechanism for recognizing ASM file naming syntax is provided. At file creation, certain file attributes are permanently set, such as its protection policy (mirroring) and its striping policy. ASM files are not visible from the operating system or its utilities, but they are visible to database instances, RMAN, and other Oracle-supplied tools.

Automated Storage Management ASM Disks

The unit of storage for ASM disk groups is ASM Disks. An ASM disk name is common to all nodes of the cluster. The administrator can specify the disk name, or it will be automatically generated by ASM when a disk is added to a disk group. Since different hosts can use different operating system names to refer to the same ASM disk, the ASM disk name abstraction is required.

To reduce the chances of losing data in case of single disk failure, ASM provides mirroring. If disk mirroring weren’t provided, the loss of the unduplicated data from a single ASM disk would possibly damage every file in the disk group.

ASM Instances

The Oracle Database 10g introduces the ASM instance, a special Oracle instance that manages the disks in disk groups. The ASM instance must be configured and running for the database instance to access Automated Storage Management files.

ASM instances are not used to mount databases, they are simply used to coordinate data layout for database instances. However, the database instances do direct I/O to disks in disk groups. I/O doesn't go through the ASM instance.

Multiple, separate database instances can share ASM disk groups for their files. A single node typically requires only a single ASM instance. The single ASM instance manages all disk groups. In a Real Application Cluster (RAC) environment, there is typically one ASM instance on each node in the RAC cluster, managing all disk groups for its node in a coordinated manner with the rest of the cluster.

All ASM management commands are directed through the ASM instance, no ASM commands are issued to any regular Oracle database instance using the ASM files.

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