The automatic recovery process

When a database is shut down during normal operation, the database server performs a checkpoint so that all the information in the database is held in the database file. This is a clean shutdown.

Each time you start a database, the database server checks whether the last shutdown was clean or the result of a system failure. If the database was not shut down cleanly, it automatically takes the following steps to recover from a system failure:

  1. Recover to the most recent checkpoint   To restore all pages to their state at the most recent checkpoint, the checkpoint log pages are copied over the changes made since the checkpoint.

    Page A, the checkpoint log copy of the page, is copied to the database file to overwrite the dirty page. The transaction log contains changes made to page A since the checkpoint.

  2. Apply changes made since the checkpoint   Changes made between the checkpoint and the system failure, which are held in the transaction log, are applied.

  3. Roll back uncommitted transactions   Any uncommitted transactions are rolled back, using the rollback logs.

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