A datafile can be stored as an ASM file, a file system file, or a raw device. Each ASM file belongs to a single disk group. A disk group may contain files belonging to several databases, and a single database may form multiple disk groups. ASM files are always spread across all the disks in the disk group.  ASM files can be created for redo log files, temporary files, RMAN files, parameter files, and data pump dump files.

ASM introduces the concept of an allocation unit (AU), the smallest contiguous disk space allocated by the ASM. The typical value for an AU is 1MB and is not user configurable. ASM does not allow physical blocks to be split across allocation units.

A disk group is a collection of disks managed as a logical unit. Within a disk group, I/Os are balanced across all the disks. Each disk group has its own file directory, disk directory, and other directories.

For better database performance, dissimilar disks should be partitioned in to separate disk groups. The redundancy characteristics are set up while defining a disk group. Please note the following:

• EXTERNAL REDUNDANCY indicates that ASM does not provide any redundancy for the disk group.
   

• NORMAL REDUNDANCY (default) enables the disk group to tolerate the loss of a single failure without data loss.
   

• HIGH REDUNDANCY provides a greater degree of protection using three-way mirroring. ASM provides near-optimal I/O balancing without any manual tuning.