Last updated on December 21st, 2015 at 06:16 am
HOW AUTOMATIC STORAGE MANAGEMENT WORKS
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
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.