The crash recovery process is a special form of recovery, which happens the first time
an Oracle database instance is started after a crash (or SHUTDOWN ABORT). In crash
recovery, the goal is to bring the datafiles to a transaction-consistent state, preserving
all committed changes up to the point when the instance failed.
系统崩溃还原进程是一种特殊的还原模式，它发生在数据库发生崩后的第一次启动的时候
（如SHUTDOWN ABORT）。系统崩溃还原中，它的目标是将数据文件保持数据的一致性
状态，预留所有的提交修改到数据库失败的时。
Like crash recovery, datafile media recovery is intended to restore database integrity.
However, there are a number of important differences between the two:
像崩溃还原样，数据文件介质还原偏向于修复数据库的完整性。但两者之间有许多重要
的不同之处：
■ Media recovery must be explicitly invoked by a user. The database will not run
media recovery on its own.
■ Media recovery applies needed changes to datafiles that have been restored from
backup, not to online datafiles left over after a crash.
■ Media recovery must use archived logs as well as the online logs, to find changes
reaching back to the time of the datafile backup.
#介质还原必须是显式通过用户调用。数据库不能自己进行介质还原。
#介质还原使用需要改变数据文件，它通过备份来修复，不是修改系统崩溃后的在线数据文件遗留需修复的。
#介质还原必须使用归档日志和在线日志，发生修改的返回到数据文件备份的时间。
Unlike the forms of recovery performed manually after a data loss, crash recovery uses
only the online redo log files and current online datafiles, as left on disk after the
instance failure. Archived logs are never used during crash recovery, and datafiles are
never restored from backup.
不如于一个数据文件丢失执行的人工还原方式，系统崩溃还原使用在线重做日志文件和
当前在线数据文件，这些文件是在实例失败后留在磁盘上。归档日志在系统崩溃还原时
是不需要用到的，并且数据文件不用从备份中修复。
The database applies any pending updates in the online redo logs to the online
datafiles of your database. The result is that, whenever the database is restarted after a
crash, the datafiles reflect all committed changes up to the moment when the haven't
said failure occurred. (After the database opens, any changes that were part of
uncommitted transactions at the time of the crash are rolled back.)
数据库应用任何未处理的在线重做日志的更新到数据库的在线数据文件。结果就是，无论
什么时候数据库在崩溃后重启，数据文件反应了所有提交修改到系统没有报失败发生时候。
（在数据库打开后，任何部份未提交事务在系统崩溃时修改将回滚）
The duration of crash recovery is a function of the number of instances needing
recovery, amount of redo generated in the redo threads of crashed instances since the
last checkpoint, and user-configurable factors such as the number and size of redo log
files, checkpoint frequency, and the parallel recovery setting.You can set parameters in
the database server that can tune the duration of crash recovery. You can also tune
checkpointing to optimize recovery time.
崩溃还原的时期是实例需要还原数量的一个功能，重做日志创建的数量在崩溃实例的REDO线程
自从最后一次检查点，用户配置因素比如重做日志文件的数量和大小，检查点的频繁度，以及并行
还原设置。你可以在数据库中设置参数，关于调试崩溃还原期间的参数。你也可以调试检查点到
一个最佳的还原时间。
Note: Crash recovery in a Real Application Cluster (RAC)
database takes place when all instances in the cluster have failed.
The related process of instance recovery takes place when some but
not all instances fail. For more information on crash and instance
recovery in the context of RAC, refer to Oracle Real Application
Clusters Quick Start.
备注：崩溃还原在RAC数据库当所有的实例在集群中都失败时发生。
相关的的实例还原进程将发生当一些实例失败时候。想了解更多的崩溃信息
和在RAC中的实例还原，可查看Oracle Real Application
Clusters Quick Start.
