How ASM Handles Disk Failures
Depending on the redundancy level of a disk group and how failure groups are
defined, the failure of one or more disks could result in either of the following:
■ The disks are first taken offline and then automatically dropped.
#2个DISKGROUP不一定必须大小相等


#磁盘组失败的定义和失败时的动作
The disk group remains mounted and serviceable, and, thanks to mirroring, all
disk group data remains accessible. Following the disk drop, ASM performs a
rebalance to restore full redundancy for the data on the failed disks.
only one failure group fails, the disk group remains mounted and serviceable, and
ASM performs a rebalance of the surviving disks (including the surviving disks in
the failed failure group) to restore redundancy for the data in the failed disks. If
more than one failure group fails, ASM dismounts the disk group.
#磁盘组中一个DISK失败 则整个磁盘视为 失败
■ A failure group is considered to have failed if at least one disk in the failure group
fails.

#1个镜像的磁盘组结构 可以忍受一个磁盘组失败(恢复失败磁盘组的剩余正常磁盘)
■ A high redundancy disk group can tolerate the failure of two failure groups. If one
or two failure groups fail, the disk group remains mounted and serviceable, and
ASM performs a rebalance of the surviving disks to restore redundancy for the
data in the failed disks. If more than two failure groups fail, ASM dismounts the
disk group.

#2个镜像的磁盘组结构 可以忍受2个磁盘组失败(恢复失败磁盘组的剩余正常磁盘)
■ An external redundancy disk group cannot tolerate the failure of any disks in the
disk group. Any kind of disk failure causes ASM to dismount the disk group.




失败磁盘中的被恢复的失败磁盘需要手动操作（ALTER DISKGROUP...ADD DISK）才能加入磁盘组 ，磁盘组(ALTER DISKGROUP...MOUNT)也是一样
When ASM drops a disk, the disk is not automatically added back to the disk group
when it is repaired or replaced. You must issue an ALTER DISKGROUP...ADD DISK
command to return the disk to the disk group. Similarly, when ASM automatically
dismounts a disk group, you must issue an ALTER DISKGROUP...MOUNT command
to remount the disk group


#尽量不要设置 数量少的大（磁盘个数多）磁盘组
假设 1个磁盘组有 16个磁盘 2个控制器件:因为一个磁盘失败就导致整个组失败 ，失败概率较大，不如分成2个FAILURE 组（NORMAL REBUNDAY）

#如何计算ASM磁盘组的可用空间；

当磁盘组中的某些磁盘失败后，磁盘组需要有额外的空间(v$ask_diskgroup字段REQUIRED_MIRROR_FREE_MB)来恢复 那些失败的磁盘数据()

select name, type, total_mb, free_mb, required_mirror_free_mb,
usable_file_mb from v$asm_diskgroup;
NAME TYPE TOTAL_MB FREE_MB REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
------------ ------ ---------- ---------- ----------------------- --------------
DISKGROUP1 NORMAL 6144 3768 1024 1372
The REQUIRED_MIRROR_FREE_MB column shows that 1 GB of extra capacity must be
available to restore full redundancy after one or more disks fail. Note that the first
three numeric columns in the query results are raw numbers. That is, they do not take
redundancy into account. Only the last column is adjusted for normal redundancy.
Notice that:
FREE_MB - REQUIRED_MIRROR_FREE_MB = 2 * USABLE_FILE_MB
or
3768 - 1024 = 2 * 1372 = 2744