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 three