首先说下实例恢复的定义：

Instance recovery is the process of applying records in the online redo log to data files to reconstruct changes made after the most recent checkpoint. Instance recovery occurs automatically when an administrator attempts to open a database that was previously shut down inconsistently.

Oracle Database performs instance recovery automatically in the following situations:

The SMON background process performs instance recovery, applying online redo automatically. No user intervention is required.

因此我们知道非一致性关闭会引发实例恢复（一致性关闭不会，参考shutdown immediate的官方定义）同时RAC节点宕机也会在一个存活节点进行实例恢复，其过程就是重构内存中的脏块并提交，同时对未提交的做出回滚，这个过程由smon后台进程负责。

这里上一幅官网的图：

图：Basic Instance Recovery Steps: Rolling Forward and Rolling Back