Crash Recovery 和 Media Recovery 的区别

最新推荐文章于 2025-03-10 14:52:22 发布

原创最新推荐文章于 2025-03-10 14:52:22 发布 · 1.4k 阅读

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#crash #database #transactions #parameters #parallel #forms

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本文介绍了Oracle数据库中的两种恢复机制：实例崩溃恢复与介质恢复。前者在实例崩溃后自动进行，确保数据文件达到一致状态；后者则需手动启动，用于恢复损坏的数据文件。两者在使用日志、触发条件等方面存在差异。

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一直自以为挺明白的看了文档后这次才清清楚楚最重要的文档一直就是概念和原理..

Backup and Recovery Basis

1.4.3 Automatic Recovery After Instance Failure: Crash Recovery

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.

Like crash recovery, datafile media recovery is intended to restore database integrity. However, there are a number of important differences between the two:

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.

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.

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.)

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.