Disk groups that are specified in the ASM_DISKGROUPS initialization parameter are mounted automatically at Oracle ASM instance startup. This process makes the disk groups available to all database instances running on the same node as Oracle ASM. The disk groups are dismounted at Oracle ASM instance shutdown. Oracle ASM also automatically mounts a disk group when you initially create it, and dismounts a disk group if you drop it.
When a disk group is mounted, a disk group number is chosen. This number may change across disk group mounts. A disk group number is not recorded in any persistent structure, but the current value can be viewed in the GROUP_NUMBER column of the V$ASM views.
When you want to mount or dismount disk groups manually, use the ALTER DISKGROUP...MOUNT or ALTER DISKGROUP...DISMOUNT statement. You can mount or dismount disk groups by name, or specify ALL. You can also specify the optional FORCE option. For information about using the FORCE option, refer to "Mounting Disk Groups Using the FORCE Option".
If you try to dismount a disk group that contains open files, the statement fails, unless you also specify the FORCE clause.
In a clustered Oracle ASM environment in RESTRICTED mode, a disk group is mounted in single-instance exclusive mode. No other Oracle ASM instance in that cluster can mount that disk group. In this mode the disk group is not usable by any Oracle ASM client. Use this mode to perform a fast rebalance.
The following SQL statement dismounts all disk groups that are currently mounted to the Oracle ASM instance:
ALTER DISKGROUP ALL DISMOUNT;
The following SQL statement mounts disk group data1:
ALTER DISKGROUP data1 MOUNT;