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这个参数是12.2新参数，用于RAC：
任何时候至少有一个active instance设置为READ-WRITE， 如果此实例挂了，则Oracle会自动关闭所有其他实例
READ-ONLY的实例不能是第一个打开数据库的实例
同一RAC不能READ-ONLY与READ-MOSTLY的实例共存

INSTANCE_MODE = { READ-WRITE(default) | READ-ONLY | READ-MOSTLY }
Modifiable: No

A READ-WRITE instance is a regular Oracle instance.
A READ-ONLY instance is an Oracle instance that can only be opened in read-only mode.
A READ-MOSTLY instance is an Oracle instance that performs very few database writes.

Unlike read/write instances, read-only instances have the following characteristics:
1)Can only open a database that has already been opened by a read/write instance
2)Disable many background processes, including the checkpoint and archiver processes, which are not necessary
3)Can mount a disabled redo thread or a thread without any online redo log