Chapter06Backup and Recovery Overview

Backup and Recovery Issues [备份与恢复所关心的问题]

• Protect the database from numerous types of failures
• Increase Mean-Time-Between-Failures(MTBF) [平均无故障时间]
• Decrease Mean-Time-To-Reconver(MTTR) [平均抢修时间]
• Minimize data loss

Categories of Failures

• Statement failure(sql 语句
  • Causes of Statement Failures
    • Logic error in an application
    • Attempt to enter invalid data into the table
    • Attempt an operation with insufficient privileges
    • Attempt to create a table but exceed allotted quota limits
    • Attempt an INSERT or UPDATE to a table,causing an extent to be allocated,but with insufficient free space available in the tablespace.
  • Resolutions
    • Correct the logcial flow of the program
    • Modify and reissue the SQL statement
    • Provide the necessary database privileges
    • Change the user`s quota limit by using the ALTER USER command
    • Add file space to the tablespace
    • Enable resumable space allocation　　
• User process failure(即：客户端程序错误)
  • Causes User process failure
    • The user performed an abnormal disconnect in the session
    • The user`s session was abnormally terminated
    • The user`s program raised an address exception,which terminated the session
  • Resolutions
    • The PMON process detects an abnormally terminated user process
    • PMON rolls back the transaction and releases any resources and locks being held by it　　
• User error
  • Cause User error
    • DROP TABLE employees;
    • TRUNCATE TABLE employees;
    • DELETE FROM employees;COMMIT;
    • UPDATE employees SET salary = salary * 1.5;COMMIT;
  • Resolution
    • Train the database user
    • Recover from a valid backup
    • Import the table from an export file
    • Use LogMiner to determin the time of error(LogMiner是Oracle提供的一项工具；)
    • Recover with a point-in-time recovery
    • Use LogMiner to perform object-level recovery
    • Use FlashBack to view and repair historical data(FlashBack是Oracle提供的一项工具；)
• Network failure 
• Instance failure
  • Instance的本质，就是一堆进程加若干个内存块，最大的内存块就是SGA
  • 最典型的Instance failure就是断电
  • SGA包含两大部分1、Data Buffer Cach；2、Redo Log Buffer Cash;
  • Redo Log File里面的内容时实时更新的，Data Filel里的内容时滞后更新的.
  • Cause User error
    • 
  • Recovery from Instance Failures
    • No special recovery action is needed from DBA
    • Start the instance
    • Wait for the "database opened" notification
    • Notify users
    • Check the alert log to determine the reason for the failure
    • SMON进程每次在Instance启动的时候，就会检测Data File与Redo Log File 是否是synchronized,如果不同步，说明需要将Redo Log File里面的内容更新到Data File里，也需要将Undo表空间里的数据同步更新到Data File里面，确保Data File与Redo Log File及Undo Tablespace里面的数据同步.对于Instance Failure来讲，DBA不需要进行手工干预.DBA需要做的就是重新启动Instance，然后等待Instance自我修复(nomounted->mounted->opened).这个过程是SMON进程在操作.
• Media failure(磁盘、介质、磁带错误)
  • Causes of Media Failures
    • Head crash on a disk drive
    • Physical problem in reading from or writing to database files
    • File was accidentally erased
  • Resolutions
    • The recovery strategy depends on which backup method was chosen and which files are affected.
    • If available,apply archived redo log files to recover data committed since the last backup.
• Defining a Strategy
  • Business requirements
    • Mean time to recover
    • Mean time between failure
    • Evolutionary process
  • Operational requirements
    • 24-Hour operations
    • Testing and validating backups
    • Database volatility
  • Technical considerations
    • Resources:hardware,software,manpower,and time
    • Physical image copies of the operating system files
    • Logical copies of the objects in the database
    • Database configuration
    • Transaction volume that affects desired frequency of backups
  • Disaster Recovery issues
    • How will your business be affected in the event of a major disater,such as:
      • Earthquake,flood,or file
      • Complete loss of machine
      • Malfunction of storage hardware of software
      • Loss of key personnel,for example the database administrator
    • Do you have a plan for testing your strategy periodically?
  • Management concurrence
  • Summary
    • In this lesson,you should have learned how to:
      • Evaluate potential failures in your enviroment
      • Develop a strategy dicated by business,operational,and technical requirements
      • Consider a test plan for a backup and recovery strategy