The Automatic Storage Management (ASM) instance is a special Oracle instance that manages the disks in disk groups. The ASM instance must be configured and running for the database instance to access Automatic Storage Management files. This configuration is done automatically if the Database Configuration Assistant was used for database creation. ASM is implemented as a special kind of Oracle instance, with its own System Global Area (SGA) and background processes. The ASM instance typically has a much smaller SGA than a database instance.
ASM instances cannot mount databases. The ASM instances simply coordinate data layout for database instances. Database instances do direct I/O to disks in disk groups without going through an ASM instance.
Multiple and separate database instances can share disk groups for their files. On a single node, there is typically a single ASM instance on the node, which manages all disk groups. In a Real Application Clusters environment, there is typically one Automatic Storage Management instance on each node managing all disk groups for its node in a coordinated manner with the rest of the cluster.
An Oracle instance that mounts ASM disk groups and performs management functions necessary to make ASM files available to database instances. ASM instances do not mount databases.