How to remove Disks from Disk Group

=> Find out the group number and name :

SQL> select group_number, name from v$asm_diskgroup ;

GROUP_NUMBER NAME
------------ ------------------------------
1 DATA
2 RECOVERY
3 GRID

=> Find out the name of the disk belonging to GROUP_NUMBER=3 which is GRID Disk Group.

SQL> select DISK_NUMBER, name, failgroup, group_number from v$asm_disk where group_number=3 order by name ;

DISK_NUMBER NAME FAILGROUP GROUP_NUMBER
----------- -------------- ---------------------- ------------
0 ASM2_VMAX00639 ASM2_VMAX00639 3
1 ASM2_VMAX0063A ASM2_VMAX0063A 3

=> so from above, there are two disks belonging to GRID diskgroup, now we'll remove one of the disks from the diskgroup
=> Drop the Disk from diskgroup named GRID

SQL> alter DISKGROUP GRID drop disk ASM2_VMAX00639 ;


=>You can check the re-balance progress using below SQL

SQL> select * from v$asm_operation;