For Oracle ASM to mirror files, specify the redundancy level as NORMAL REDUNDANCY (2-way mirroring by default for most file types) or HIGH REDUNDANCY (3-way mirroring for all files).
Specify EXTERNAL REDUNDANCY if you do not want mirroring by Oracle ASM. For example, you might choose EXTERNAL REDUNDANCY to use storage array protection features.After a disk group is created, you cannot alter the redundancy level of the disk group.To change the redundancy level, you must create another disk group with
the appropriate redundancy and then move the files to the new disk group.Oracle recommends that you create failure groups（故障组） of equal size to maintain space balance and even distribution of mirror data.
关于ASM中故障组，磁盘组，冗余级别的关系：
这几个概念是非常容易混淆的，先简单梳理一下概念：
冗余级别：External Redundancy、Normal RedunDancy、High Redundancy 这三个级别；其中，External级别，对于每个extend（asm的最小数据单元）只存储一份，没有 做冗余，这种情况一般是在硬件存储级别有做Riad；Normal冗余级别，每个extend只有一份镜像冗余；High冗余级别，每个extend有两份镜像冗余。
磁盘组：两个或两个以上的asm磁盘可以组成一个磁盘组。
故障组：实际上故障组就是ASM DISK的一个逻辑组合,如果不指定哪些ASM DISK属于哪个FAILGROUP,那么可以理解每个ASM DISK都是一个FAILGROUP。
下面综合描述这些关系，讲的非常清晰（http://www.itpub.net/thread-1347371-1-1.html）：
故障组主要是对 asm extend mirror 位置的强化。
例如4个asm disk， 作了1个nomal 冗余 的dg（这时有4个故障组，一个磁盘组）， 那么对于任何一个asm extend（asm 以extend 为单位进行冗余）都有可能在其余的3个asm disk 上。如果我们作成了2个failgroup （gp1: disk1 disk2, gp2 disk3 disk4) （这时有2个故障组，一个磁盘组）那么对于任何一个在disk1，disk2 上的extend 只能mirror 到disk3 或者4 了，反之亦然。
——只有normal以上的级别，设置故障组才有意义，对于外部冗余不需要设置故障组。（如果没有显式指定故障组，默认的每个asm磁盘都是一个故障组）
参考链接：http://www.itpub.net/thread-1347371-1-1.html
The results from the following query show capacity metrics for a normal redundancy
disk group that consists of six 1 GB (1024 MB) disks, each in its own failure group:
SQL> 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
------------ ------ ---------- ---------- ----------------------- --------------
DATA 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. 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. In the query
output example for the data disk group, the calculation is as follows:
(FREE_MB - REQUIRED_MIRROR_FREE_MB) / 2 = USABLE_FILE_MB
(3768 - 1024) / 2 = 2744 / 2 = 1372
2、Voting Disk and OCR
Voting files manage information about node membership. OCR is a system that manages cluster and Oracle Real Application Clusters (Oracle RAC) database
configuration information.
A quorum failure group is a special type of failure group and disks in these failure groups do not contain user data. A quorum failure group is
not considered when determining redundancy requirements in respect to storing user data.However, a quorum failure group counts when mounting a disk group
关于这个讨论，参考http://www.itpub.net/thread-1443109-1-1.html 这里讲的很详细。
