Restoring with Media Errors in VS .NET

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Restoring with Media Errors
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The most difficult problem to overcome during a restore is having media that has been damaged. In previous SQL Server versions, media damage always made a bad situation even worse. Damage to the backup media is rarely detected before a backup begins. And after a restore starts, it wipes out everything that had previously existed in the database. If the restore operation were to abort, you would be left with a completely invalid database. Unfortunately, this is what occurs when the backup media is damaged. SQL Server 2005 now has an option for the RESTORE command that enables SQL Server to skip damaged media sectors and finish the restore operation. By using the WITH CONTINUE_AFTER_ERROR option, damaged media sectors are skipped, and any readable parts of the media will be restored.
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Backing Up, Restoring, and Moving a Database
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Although the restore operation completes, it does not guarantee that the database will be usable or that it will even contain any data. At the completion of a RESTORE operation in which media errors have occurred, the database is placed in emergency mode. In this mode, you can make a connection to the database and execute SELECT statements, but you cannot make changes to the data. If you determine that the database is intact and operational, you can change the state to allow normal operations. In a worst-case scenario, any intact data can be extracted from the database. Although this solution isn t perfect, it is better than nothing. That is why this feature is more popularly known as a best-effort restore.
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Validating a Backup
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You have performed several backups, but how do you know the backups are usable The only way to guarantee that a backup is usable is to restore it and verify all the data. This process can be very time-consuming and is rarely practical. However, SQL Server provides a way to verify the integrity of a backup. Although not the same as actually restoring a database, it provides a very thorough check of the backup integrity. You use the following command to verify a backup s integrity:
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RESTORE VERIFYONLY FROM <backup_device> [ ,...n ]
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When you execute this command, SQL Server checks the media header to ensure that it is intact. It then verifies the backup checksum, reads the internal page chains, and recalculates the backup checksum for comparison. A variety of checks are performed to ensure that the backup is intact. However, SQL Server does not check the actual data structures in the backup.
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Previous versions and backup verification
Previous versions of SQL Server also had a RESTORE VERIFYONLY command, which checked the media header and then returned a success or an error. The entire backup set could be invalid, and every other sector on the media could be bad. But as long as the media header was intact, it would return a success. This behavior effectively rendered this command worthless in previous versions because the command didn t actually check anything useful. So, everyone recommended not using this command. However, SQL Server 2005 now performs the necessary checks, so you should execute this command every time you perform a backup.
Lesson 2: Restoring a Database
PRACTICE
Restoring the AdventureWorks Database
Using the backups from Lesson 1, restore the AdventureWorks database to the current point in time. 1. If necessary, launch SSMS, connect to your SQL Server instance, and open a new query window. 2. Change the context to the master database. 3. Back up the tail of the log by executing the following command:
BACKUP LOG AdventureWorks TO DISK = C:\TEST\AWTAIL.TRN
4. Execute the following RESTORE commands to restore the AdventureWorks database:
RESTORE RESTORE RESTORE RESTORE DATABASE AdventureWorks DATABASE AdventureWorks LOG AdventureWorks FROM LOG AdventureWorks FROM FROM FROM DISK DISK DISK = C:\TEST\AW.BAK WITH NORECOVERY DISK = C:\TEST\AWDIFF1.BAK WITH NORECOVERY = C:\TEST\AW2.TRN WITH NORECOVERY = C:\TEST\AWTAIL.TRN WITH RECOVERY
5. If you have performed both exercises from Lesson 1, the AdventureWorks database should have only a single filegroup, and the dbo.t1 table should not exist. 6. If you did not perform the filegroup backup from Lesson 1, you need to verify that the AdventureWorks database contains all the changes that were made.
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