ASM Instance

ASM instance is a special kind of Oracle Instance which has only SGA and Background process and no Physical Files except parameter file. It manages the disk group activity. Each server has an ASM instance that manages the disk groups for that server. If the server has more than one database a single ASM instance can manage the disk group for all the databases. In an RAC configuration each node has a separate ASM instance that manages disk groups for that node.

ASM breaks the files into multiple extents and spreads the extents across all of the disks available in the disk group. Usually the extent is 1MB in size. The ASM instance creates an extent map, which has a pointer to each 1MB extent. When the database instance wants to create or open the datafile, it messages the ASM instance and ASM instance returns the extent map for that file. The database instance then performs the I/O directly with the datafile.

ASM instance must be started before the database instance starts and must be running as long as the database instance runs.

Initialization Parameters for ASM instance

Unlike database instance, ASM instance does not have physical files and can be started with few initialization parameters. The following are the required parameters.

*.instance_type=asm
*.cluster_database=TRUE
*.instance_number=1
*.remote_login_passwordfile=EXCLUSIVE
*.asm_diskgroups=ASMIND,ASMDB,ASMFLASH
*.background_dump_dest=’/…/’
*.core_dump_dest=’/…/’
*.user_dump_dest=’/…/’

The following size is used for most configurations

*.Shared_pool_size=128MB
*.Large_pool_size= 12MB
*.Db_cache_size=64MB

From Oracle 11g, there is no need to set bdump,cdump,udump. Instead set the following Automatic Diagnostic Repository parameter,

*.diagnostic_dest=’/…/’

Starting ASM instance

There are four modes to start asm instance they are,

SQL> Startup Nomount; -- starts the asm instance.
SQL> Startup Mount; -- mounts the disk groups
SQL> Startup Force; -- shuts down asm and restarts.
SQL> Startup Restrict; -- prevents oracle database from connecting to asm.

Stopping ASM instance

SQL> Shutdown;

Shutting down the ASM instance shuts down any oracle databases connected to that ASM instance in the same mode. When you issue shutdown normal, immediate or transactional ASM instance waits for the corresponding oracle databases to finish all SQL operations before shutting down.

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