OCP-1Z0-053-V12.02-649题

649.Examine the CREATE DISKGROUP command used to create a disk group:

SQL> CREATE DISKGROUP misc EXTERNAL REDUNDANCY

DISK 'ORCL: FRA3' NAME misc1, 'ORCL: FRA4' NAME misc2;

In which situation would you use this method of disk group creation?

A. When two-way disk mirroring is required for the allocation units

B. When three-way disk mirroring is required for the allocation units

C. When using hardware mirroring or RAID

D. When disk mirroring is required for the Automatic Storage Management (ASM) disks

Answer: C

答案解析：

参考：http://blog.csdn.net/rlhua/article/details/12972983

CREATE DISKGROUP语句，该语句的语法如下： 

CREATE DISKGROUP diskgroup_name 

  [ { HIGH | NORMAL | EXTERNAL } REDUNDANCY ] 

  [ FAILGROUP failgroup_name ] 

  DISK [ NAME disk_name ] [ SIZE size_clause ] [ FORCE | NOFORCE ] ...; 

•首先要指定的就是磁盘组名称(diskgroup_name)；  

•指定冗余度，有三个选择：HIGH(高度冗余>三路)、NORMAL(标准冗余--双路)和EXTERNAL(外部存储冗余)；   

•选择是否指定FAILGROUP(如果选择非external则必须指定)；   

•指定该磁盘组中的成员(对应的LUN)，在指定成员时一般ASM能够自动检测出磁盘的容量，不过如果DBA基于某些方面的考虑，希望限制ASM使用的空间的话，也可以在指定成员过程中，顺便指定大小(只要指定的大小不超出磁盘实际容量)，在添加成员时，ASM也会自动检查磁盘头以确定该磁盘是否被加入到其它的磁盘组中，当发现该盘已加入其它磁盘组的话，你可以通过FORCE选项来强制修改该盘所属磁盘组。 

REDUNDANCY Clause

The REDUNDANCY clause lets you specify the redundancy level of the disk group.

• NORMAL REDUNDANCY requires the existence of at least two failure groups (see the FAILGROUP clause that follows). Oracle ASM provides redundancy for all files in the disk group according to the attributes specified in the disk group templates. NORMAL REDUNDANCY disk groups can tolerate the loss of one group. 

  NORMAL REDUNDANCY is the default if you omit the REDUNDANCY clause. Therefore, if you omit this clause, you must create at least two failure groups, or the create operation will fail.

• HIGH REDUNDANCY requires the existence of at least three failure groups. Oracle ASM fixes mirroring at 3-way mirroring, with each extent getting two mirrored copies. HIGH REDUNDANCY disk groups can tolerate the loss of two failure groups.

• EXTERNAL REDUNDANCY indicates that Oracle ASM does not provide any redundancy for the disk group. The disks within the disk group must provide redundancy (for example, using a storage array), or you must be willing to tolerate loss of the disk group if a disk fails (for example, in a test environment). You cannot specify the FAILGROUP clause if you specify EXTERNAL REDUNDANCY.

  官方参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_5008.htm#sthref4362

  
  

  EXTERNAL REDUNDANCY即是ASM不会有故障组，需要自己提供别的冗余方式，如硬盘或者raid冗余