How to mount or dismount an ASM disk group in Oracle

Mount an ASM disk group to make its files available to databases, or dismount it. Run connected to the ASM instance AS SYSASM.

Dismounting a disk group makes every file on it (datafiles, redo, and so on) unavailable, so close or relocate dependent databases first. A FORCE dismount should be a last resort.

-- Mount
ALTER DISKGROUP &&dg_name MOUNT;

-- Mount every defined disk group:
-- ALTER DISKGROUP ALL MOUNT;

-- Dismount (only when no client is using it):
-- ALTER DISKGROUP &dg_name DISMOUNT;

-- Verify (read-only)
SELECT name, state, type, total_mb, free_mb
FROM   v$asm_diskgroup
WHERE  name = UPPER('&dg_name');