Automatic Storage Management (ASM)

ASM is a database file system that provides a cluster file system and volume manager capabilities for oracle datafiles that are integrated into the oracle database 10g kernel. The ASM environment provides the performance of raw I/O with the easy management of a file system.

ASM supports datafiles, log files, control files archive logs, RMAN backup sets and other oracle database file types.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 of several physical disks that are controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are know 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.

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.

Initialization Parameters and ASM Instance Creation

The initialization parameters that are of specific interest for an ASM instance are:

• 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 you intend 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.

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.