初始化参数asm_diskgroups指定了在ASM实例启动时，或执行alter diskgroup all mount语句时，被挂载的磁盘组的名字，即当ASM实例启动的时候，会挂载这个参数指定的所有磁盘组，
或当执行alter diskgroup all mount/dismount语句时，也会挂载这个参数指定的所有磁盘组。
关于此参数的值，是由ASM自动维护的，不需要人去干预，修改。
1 当成功创建一个磁盘组或是挂载一个磁盘组时，磁盘组的名字将会自动添加到此参数中。
2 当删除或卸载一个磁盘组时，磁盘组的名字也将相应地从这个参数中移除。
注意：
如果挂载或卸载使用的是alter diskgroup all mount或alter diskgroup all dismount，此参数的值将不会发生变化！
只有在挂载或卸载时执行alter diskgroup xxx mount或alter diskgroup xxx dismount时，才会影响此参数的值。
做实验如下：
SQL> select name,STATE from v$asm_diskgroup;
NAME STATE
----- --------
DATA1 MOUNTED
SQL> create diskgroup DATA2 external redundancy disk 'ORCL:VOL201','ORCL:VOL202','ORCL:VOL203','ORCL:VOL204';
Diskgroup created.
SQL> select name,STATE from v$asm_diskgroup;
NAME STATE
--------------- ---------------
DATA1 MOUNTED
DATA2 MOUNTED
SQL> show parameter asm_diskgroups;
NAME TYPE VALUE
-------------- -------- -------------
asm_diskgroups string DATA2
SQL> create pfile='/tmp/asm_20110905.ora' from spfile;
File created.
查看文件内容：
[root@rac1 tmp]# cat asm_20110905.ora
+ASM1.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM2.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM1.asm_diskgroups='DATA2'#Manual Mount
*.asm_power_limit=1
*.diagnostic_dest='/opt/rac/grid'
*.instance_type='asm'
*.large_pool_size=12M
*.remote_login_passwordfile='EXCLUSIVE'
在节点2上
SQL> alter system set asm_diskgroups='DATA2' scope=spfile sid='+ASM2';
System altered.
SQL> show parameter asm_diskgroups;
##这里手工操作以后，asm_diskgroups里面是没有信息的！！
NAME TYPE VALUE
--------------- ------------ ---------------
asm_diskgroups string
SQL> create pfile='/tmp/asm_20110905.ora' from spfile;
File created.
查看生成的文件！
[root@rac2 tmp]# cat asm_20110905.ora
+ASM1.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM2.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM1.asm_diskgroups='DATA2'#Manual Mount
+ASM2.asm_diskgroups='DATA2'--手工加上的！
*.asm_power_limit=1
*.diagnostic_dest='/opt/rac/grid'
*.instance_type='asm'
*.large_pool_size=12M
*.remote_login_passwordfile='EXCLUSIVE'
在节点2上检查DATA2的状态：
grid@rac2:/home/grid>crsctl status resource ora.DATA2.dg
NAME=ora.DATA2.dg
TYPE=ora.diskgroup.type
TARGET=ONLINE , OFFLINE
STATE=ONLINE on rac1, OFFLINE 在节点2上未启动！！
grid@rac2:/home/grid>sqlplus "/as sysasm"
SQL*Plus: Release 11.2.0.1.0 Production on Mon Sep 5 14:18:25 2011
Copyright (c) 1982, 2009, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Real Application Clusters and Automatic Storage Management options
SQL> select name,STATE from v$asm_diskgroup;
NAME STATE
----------------------
DATA1 MOUNTED
DATA2 DISMOUNTED
SQL> alter diskgroup DATA2 MOUNT;
Diskgroup altered.
SQL> show parameter asm_diskgroups; ###mount DATA2磁盘之后，asm_diskgroups 显示了数据DATA2
NAME TYPE VALUE
---------------- -------- -------------------
asm_diskgroups string DATA2
SQL> create pfile='/tmp/asm2_1.ora' from spfile;
File created.
SQL> exit
[root@rac2 tmp]# cat asm2_1.ora
+ASM1.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM2.__oracle_base='/opt/rac/grid'#ORACLE_BASE set from in memory value
+ASM1.asm_diskgroups='DATA2'#Manual Mount
+ASM2.asm_diskgroups='DATA2'#Manual Mount
##oracle 自己添加上去的！！！不过要注意 Manual Mount 提示，手动mount。
*.asm_power_limit=1
*.diagnostic_dest='/opt/rac/grid'
*.instance_type='asm'
*.large_pool_size=12M
*.remote_login_passwordfile='EXCLUSIVE'
这里11.2.0.1 有一个bug 8663101 ！和磁盘的mount 状态有关！
