Note_Tech

All technological notes.

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Revovery - Instance Recovery

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• Revovery - Instance Recovery
  • Instance Recovery
    • Instance Failure
    • Instance Recovery
    • Phases of Instance Recovery

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Instance Recovery

Instance Failure

• Typical Causes & Possible Solutions:

• Instance failure:

  • occurs when the database instance is shut down before synchronizing all database files.
  • can occur because of
    • hardware failure
    • software failure
    • the use of the emergency SHUTDOWN ABORT and STARTUP FORCE shutdown commands.

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Instance Recovery

• Automatic instance or crash recovery:

  • caused by attempts to open a database whose files are not synchronized on shutdown
  • Uses information stored in redo log groups to synchronize files
  • Involves two distinct operations:
    • Rolling forward: Redo log changes (both committed and uncommitted) are applied to data files.
    • Rolling back: Changes that are made but not committed are returned to their original state.

• Instance Recovery only needs the instance to be started normally.

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Phases of Instance Recovery

1. Startup instance (data files are out of sync)

   • The instance mounts the control files and then attempts to open the data files.

2. Roll forward (redo):

   • For an instance to open a data file, the system change number (SCN) contained in the data file’s header must match the current SCN that is stored in the database’s control files.
   • If the numbers do not match, the instance applies redo data from the online redo logs, sequentially “redoing” transactions until the data files are up to date, to bring the database up to the state as of the time of failure.
   • includes Committed and uncommitted data in files
3. Database opened
   • After all data files have been synchronized with the control files, the database is opened, and users can log in.

4. Roll back (undo):

   • After the database has been opened, those uncommitted transactions are rolled back.
   • At the end of the rollback phase of instance recovery, the data files contain only committed data.

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