結論：
如果一個diskgroup有一個failgroup offline，超出disk_repair_time定義的時間後，asm會對該failgroup做drop操作。
如果drop之後剩餘的failgroup少於冗餘策略的最低要求（normal爲2，high爲3）或者剩餘空間不足以滿足冗餘的空間
需求，也就是rebalance無法正常進行，就會出現forcing狀態的磁盤。
如果有forcing狀態的盤，使用alter添加磁盤時要指定name, name是forcing狀態的盤原先的name。
例如ssddg的磁盤ssddg_0001盤被drop掉了，state爲forcing，用下面命令恢復磁盤組之後，SSDDG_0001狀態變爲normal:
alter diskgroup ssddg add failgroup rac2 disk '/dev/raw/raw3' name SSDDG_0001 force;實驗過程：
實驗對象爲一個有3個failgroup的磁盤組，實驗需要，把disk_repair_time設置爲5min：
SQL> select name,failgroup,path from v$asm_disk where group_number=(select group_number from v$asm_diskgroup where name='SSDDG');
NAME FAILG PATH
--------------- ----- --------------------------------------------------------------------------------
SSDDG_0001 RAC2 /dev/raw/raw3
SSDDG_0000 RAC1 /dev/raw/raw2
SSDDG_0001_0 RAC3 /dev/raw/raw5
ASMCMD> lsattr -G ssddg -lm
Group_Name Name Value RO Sys
SSDDG access_control.enabled FALSE N Y
SSDDG access_control.umask 066 N Y
SSDDG au_size 1048576 Y Y
SSDDG cell.smart_scan_capable FALSE N N
SSDDG compatible.asm 11.2.0.3.0 N Y
SSDDG compatible.rdbms 11.2.0.3 N Y
SSDDG content.type data N Y
SSDDG disk_repair_time 5m N Y
SSDDG idp.boundary auto N Y
SSDDG idp.type dynamic N Y
SSDDG sector_size 512 Y Y
磁盤組裏只有很少的數據：
ASMCMD> lsdg
State Type Rebal Sector Block AU Total_MB Free_MB Req_mir_free_MB Usable_file_MB Offline_disks Voting_files Name
MOUNTED EXTERN N 512 4096 1048576 20480 18245 0 18245 0 N DATADG/
MOUNTED EXTERN N 512 4096 1048576 2048 1653 0 1653 0 Y OCRVOTE/
MOUNTED NORMAL N 512 4096 1048576 24576 24237 8192 8022 0 N SSDDG/
實驗一、
手動把SSDDG_0001_0這塊盤offline：
SQL> select name,failgroup,path,mode_status,state from v$asm_disk where group_number=(select group_number from v$asm_diskgroup where name='SSDDG');
NAME FAILG PATH MODE_STATUS STATE
--------------- ----- -------------------------------------------------------------------------------- -------------- ----------------
SSDDG_0001_0 RAC3 OFFLINE NORMAL
SSDDG_0001 RAC2 /dev/raw/raw3 ONLINE NORMAL
SSDDG_0000 RAC1 /dev/raw/raw2 ONLINE NORMAL
5分鐘之後盤被drop
NAME FAILG PATH MODE_STATUS STATE
--------------- ----- -------------------------------------------------------------------------------- -------------- ----------------
_DROPPED_0001_S RAC3 OFFLINE FORCING
SDDG
SSDDG_0001 RAC2 /dev/raw/raw3 ONLINE NORMAL
SSDDG_0000 RAC1 /dev/raw/raw2 ONLINE NORMAL
drop之後的最終結果：
NAME FAILG PATH MODE_STATUS STATE
--------------- ----- -------------------------------------------------------------------------------- -------------- ----------------
SSDDG_0001 RAC2 /dev/raw/raw3 ONLINE NORMAL
SSDDG_0000 RAC1 /dev/raw/raw2 ONLINE NORMAL
結論：offline的盤被drop之後，磁盤組還能保持2個failgroup，空間也足夠normal冗餘，則盤的信息不再有記錄。
實驗二、在實驗一基礎上繼續offline一個failgroup，drop之後的最終結果：
NAME FAILG PATH MODE_STATUS STATE
--------------- ----- -------------------------------------------------------------------------------- -------------- ----------------
_DROPPED_0002_S RAC2 OFFLINE FORCING
SDDG
SSDDG_0000 RAC1 /dev/raw/raw2 ONLINE NORMAL
實驗三、環境同實驗一，在ssddg上創建10G的數據文件，如果drop掉一個failgroup，diskgroup的空間不能滿足normal冗餘。將一個failgroup offline ，drop之後的最終結果：
NAME FAILG PATH MODE_STATUS STATE
--------------- ----- -------------------------------------------------------------------------------- -------------- ----------------
_DROPPED_0002_S RAC3 OFFLINE FORCING
SDDG
SSDDG_0000 RAC1 /dev/raw/raw2 ONLINE NORMAL
SSDDG_0001 RAC2 /dev/raw/raw3 ONLINE NORMAL
asmcmd lsop的結果：
[grid@node1 ~]$ asmcmd lsop
Group_Name Dsk_Num State Power EST_WORK EST_RATE EST_TIME
SSDDG REBAL ERRS 1
alert日誌內容：
Thu Mar 31 15:31:00 2016
ERROR: ORA-15041 thrown in ARB0 for group number 3
Errors in file /oracle/11.2.0/grid/log/diag/asm/+asm/+ASM1/trace/+ASM1_arb0_22412.trc:
ORA-15041: diskgroup "SSDDG" space exhausted
Thu Mar 31 15:31:00 2016
NOTE: stopping process ARB0
NOTE: rebalance interrupted for group 3/0xf7e87524 (SSDDG)