Automatic
Storage Management has a special-purpose Oracle instance, called the ASM
instance, dedicated to managing disk group activity. The ASM instance manages
and communicates the map as to where each file extent resides. It also controls
the process of rebalancing the placement of the extents when the storage
allocation is changed. As an ASM instance uses only about 64-MB for its system
global area, it requires a relatively small amount of system resource. In a RAC
configuration, an ASM instance on each node in the cluster manages all disk
groups for that node, in coordination with the other nodes in the cluster.
The
ASM instance creates an extent map which has a pointer to each 1MB extent of
the data file is located. When a database instance creates or opens a database
file that is managed by ASM, the database instance messages the ASM instance
and ASM returns an extent map for that file. From that point the database
instance performs all I/O directly to the disks unless the location of that
file is being changed. Three things might cause the extent map for a database
instance to be updated: 1) Rebalancing the disk layout following an
storage
configuration change (adding or dropping a disk from a disk group), 2) Opening
of a new database file and 3) extending an existing database file when a tablespace
is enlarged.
An ASM
instance cannot mount a database; it mounts disk groups and the database
instance mounts the database. An ASM instance must be started before a database
instance can access files in disk groups. Multiple and separate database
instances can share disk groups for their files. On a single node, a
single
ASM instance typically manages all disk groups. In a Real Application Cluster
environment, each node typically has one ASM instance that manages all disk
groups for its node in a coordinated manner with the rest of the cluster. All
ASM management commands, such as creating disk groups or adding or dropping
disks, must be directed to the ASM instance, and not to the database instance
using the ASM Files. The status of disk groups and ASM attributes can be viewed
through Enterprise Manager or through V$views in the ASM instance.
