When starting up your ASM instance, you receive the following error: 
What is the cause of the error?
A. The ASM_DISKGROUPS parameter is configured for three disk groups: DGROUP1, DGROUP2, and DGROUP3.The underlying disks for these disk groups have apparently been lost.
B. The format of the ASM_DISKGROUPS parameter is incorrect. It should reference the disk group numbers, not the names of the disk groups
C. The ASM_POWER_LIMIT parameter is incorrectly set to 1. It should be set to the number of disk groups being attached to the ASM instance.
D. The ASM_DISKSTRING parameter is not set; therefore disk discovery is not possible.
E. There is insufficient information to solve this problem.
The ASM_DISKSTRING parameter is not set correctly.When the ASM instance is started,it will use the ASM_DISKSTRING to do ASM disk discovery.Correct the ASM_DISKSTRING parameter,and restart the instance to correct the problem.
ASM_DISKSTRING 是一个与操作系统相关的值，ASM 使用它来限制搜索时考虑的磁盘集即是设置 ASM启动时检查的磁盘，该选项可以同时指定多个值，并且支持通配符。比如只检查/DEV/SD*。默认值为空字符串，为空的话，表示 ASM 将查找系统中所有 ASM 拥有读写权限的设备。这在大多数情况下就足够了。如上所示的约束性更强的值可以减少 ASM 执行搜索所需的时间，从而提高磁盘组装载次数。
从图上可以，他的 ASM_DISKSTRING 没有设置。