A disk group comprises of a set of Automatic Storage Management (ASM) disks that are managed together as a logical unit. Rather than managing hundreds, possibly thousands of files, the administrators create and administer the disk group, by using ASM. The data structures in a disk group are self contained and consume some of the disk space in a disk group. ASM disks can be added or dropped from a disk group while the database is running. ASM rebalances the data to ensure an even I/O load to all disks in a disk group even when the configuration of the disk group changes.
Any single ASM file is contained in a single disk group. However, a disk group can contain files belonging to several databases, and a single database can use storage from multiple disk groups. One or more disk groups can be specified as the default disk group for files created in a database. ASM ensures that a file is evenly spread across all disks in a disk group when the file is allocated.
There are three types of disk groups: normal redundancy, high redundancy, and external redundancy. With normal and high redundancy, ASM provides redundancy for all files in the disk group according to the attributes specified in the disk group templates. High redundancy provides a greater degree of protection. With external redundancy, ASM does not provide any redundancy for the disk group. The underlying disks in the disk group should provide redundancy (for example, using a storage array), or the user must be willing to tolerate loss of the disk group if a disk fails (for example, in a test environment).