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Types of ASM Disk Groups

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

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. The redundancy level or type is specified at the time of creating the disk groups.

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. More user-friendly alias names can be created 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 5.2.
























Table 5.2:  Contents of the v$osm_alias View

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.

ASM Templates

Collections of attributes used by ASM during file creation are known as templates. Templates are used to simplify ASM file creation by mapping complex file attribute specifications into a single named object (template). Each Oracle file type has its own default template. Each disk group contains its own set of definition templates. Template names only have to be unique within a single ASM disk group, a template of the same name can exist in different disk groups with each separate template having their own unique properties.

Administrators can change the attributes of the default templates or add their own templates. This lets an administrator specify the appropriate file creation attributes as a template. However, if a DBA needs to change an ASM file attribute after a file has been created, then the file must be copied using RMAN into a new file created with a different template that contains the new attributes.

ASM Disks

The unit of storage for ASM disk groups is ASM Disks. ASM disks can be entire physical disks, LUN(s) from a storage array, partition of a LUN or pre-created files in a NAS filer. 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.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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