2.2.5 Mounting or Dismounting an Oracle ASM Disk Group

A disk group must be mounted by Oracle ASM before a database can access the files in the disk group.

Mounting a disk group requires discovering all of the disks and locating the files in the disk group. When an Oracle ASM starts, the disk groups mentioned in the ASM_DISKGROUPS instance parameter are automatically mounted.

Additionally:

• To mount a disk group, you can use the SQL ALTER DISKGROUP command with the MOUNT option.
  You can use the FORCE option in conjunction with the ALTER DISKGROUP ... MOUNT command to mount a disk group even if a disk is unavailable. However, this compromises redundancy in the disk group.
• To dismount a disk group, you can use the SQL ALTER DISKGROUP command with the DISMOUNT option.