ACTIVE的日志组一般是切换前正在使用的CURRENT的日志组，切换后由CURRENT变为ACTIVE意味着如果实例失败，SMON进行实例恢复时仍然需要这个日志组进行恢复。当ALTER SYSTEM CHECKPOINT时，表示强制检查点更改至最新，所以它变为INACTIVE。
checkpoint后，数据文件记录checkpoint scn，这个是recover的起点。
你checkpoint以前的redo就不需要在apply到 crash recover中了，当然就减少了，crash recover的时间~
checkpoint后，数据文件记录checkpoint scn，这个是recover的起点。
你checkpoint以前的redo就不需要在apply到 crash recover中了，当然就减少了，crash recover的时间~
crash recovery
The automatic application of online redo records to a database after either a single-instance database crashes or all instances of an Oracle Real Applications Cluster configuration crash. Crash recovery only requires redo from the online logs: archived redo logs are not required.
In crash recovery, an instance automatically recovers the database before opening it. In general, the first instance to open the database after a crash or SHUTDOWN ABORT automatically performs crash recovery.
The automatic application of online redo records to a database after either a single-instance database crashes or all instances of an Oracle Real Applications Cluster configuration crash. Crash recovery only requires redo from the online logs: archived redo logs are not required.
In crash recovery, an instance automatically recovers the database before opening it. In general, the first instance to open the database after a crash or SHUTDOWN ABORT automatically performs crash recovery.
Oracle 在实例故障（instance failure）之后能自动地执行崩溃恢复（crash recovery）及实例恢复（instance recovery）。崩溃恢复及实例恢复能够在实例故障发生后将数据库恢复到满足事务一致性（transaction-consistent）的状态。崩溃恢复的定义是：单实例系统发生崩溃或 Oracle RAC 系统中所有实例发生崩溃后对数据库进行的恢复。与之相对，实例恢复的定义是：Oracle RAC 系统中正常实例对故障实例进行的恢复。