Lecture7 - Lecture 7 on Database Recovery This lecture...

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01/11/12 Lecture 7 on Database Recovery This lecture covers the backup recovery of a relational database in case of system failure such that the original relational database can be reconstructed from a copy of a saved relational database beforehand. Similarly, it can also be saved from an incomplete transaction etc.
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01/11/12 Types of database system failures Action failure Transaction failure System failure Media failure
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01/11/12 Database Recovery Techniques Dumps/Checkpoints Duplication Generation method
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01/11/12 Dumps/checkpointing Recovery procedure used will be in a form of dumping plus logging for most on-line database systems. This technique entails making safe copies of the database or part of it, at intervals termed checkpoints; and accumulating a separate log-file of copies of transactions affecting the database. During recovery from a system failure, the database is restored by reapplying transactions followed the checkpoint to the safe copy of the database.
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01/11/12 Elements in dumping/ Checkpointing Elements Database dump Before/After images Transaction log Success unit Checkpoint Write-ahead log Recovery Procedures Reprocessing Roll-forward Roll-back Roll-forward with roll-back Delayed updating
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01/11/12 Success unit It is a unit of processing, and during the execution of this the data it uses must not be updateable by any other process. For example, Undo(Undo(Undo…(x)))=Undo(x) for all x The result of nested undo(x)s is the same as an undo(x). That is, an undo may be interrupted due to system failure, but will be restarted and continue with another nested undo until it completes the undo, during which all of its data it uses must not be updateable by any other process.
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01/11/12 Duplication Duplication is applied where immediate recovery from a hardware fault is required. The method is to keep two identical copies of the database, and apply all updates to both simultaneously. If one of the copy is damaged, the recovery procedure is to set a few status switches to the other one and operate on it. Then the damaged copy will be recovered and brought forward to current status.
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01/11/12
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01/11/12 Generation method This is to redo all transactions to the database from previous generations. For each generation, the database is dumped onto archive storage and all the transactions are logged. The dump should be taken when the database is static, that is, when no transactions are currently active, and all updates have been forced out to secondary storage. This is to prevent the dump from containing uncommitted changes that will have to be undone if the dump is used in the recovery process.
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Generation method recovery procedure Load the database on the new device from the most recent archive dump.
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Lecture7 - Lecture 7 on Database Recovery This lecture...

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