One ASM DISK GROUP contains all of the available disks of the server. Need to re-configure the current DISK GROUP to break it down into three different ASM disk groups.
注意事项：
1). A DISK GROUP with single disk is possible only if the redundancy of diskgroup is EXTERNAL.
2). We can n't change the redundancy of a diskgroup once it is created
3). By using external redundancy we are depending on external mechanisms (like RAID) for fault tolerance.
操作步骤：
1) Check the redundancy of diskgroup and Identify the group number.
2) Identify the physical disk names.
3) Drop the identified disks from existing diskgroup.
4) Create new diskgroups.
具体操作如下：
1) Check the redundancy of diskgroup and Identify the group number.
ASM:SQL>SELECT GROUP_NUMBER,TYPE FROM V$ASM_DISKGROUP WHERE NAME='DATA';
GROUP_NUMBER TYPE
------------ -------
1 EXTERN
(Here EXTERN indicates the Diskgroup DATA is created with redundancy external).
2) Identify the DISK Name and Path.
ASM:SQL > SELECT NAME,PATH FROM V$ASM_DISK WHERE GROUP_NUMBER=1
NAME PATH
----------- ----------
DATA_0001 /dev/raw/raw1
DATA_0002 /dev/raw/raw2
DATA_0003 /dev/raw/raw3
3) Drop the identified disks from existing diskgroup.
ASM:SQL> ALTER DISKGROUP DATA DROP DISK DATA_0002
ASM:SQL> ALTER DISKGROUP DATA DROP DISK DATA_0003
4) Create new two diskgroups.
create diskgroup DATA_A external redundancy disk '/dev/raw/raw1'
create diskgroup DATA_B external redundancy disk '/dev/raw/raw2'
Note: The steps provided here assume that there is a disk group DATA with external redundancy contains three disks. This need to be break down to three seperate diskgroups. We depend on external redundancy when the defined disks have some external means for fault tolerence(eg RAID),ie Oracle will not do any mirroring for these diskgroups
