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Tibor Karaszi

What does RESTORE do?

I was replying to a newsgroup post today, explaining the restore process. I find some confusion in various places about what actually happens during restore, and hopefully below can help a bit:

Fact: All backups contains log records. A log backup contains only of log records (more later on bulk-logged recovery). The different types of database backup contain the log records that was produced while the backup was running - in addition to data pages.
 
The restore command can perform several things:
  1. Delete the existing database. This happens if you specify the REPLACE option.
  2. Create the existing database. This happens if the database name you specify in the RESTORE command doesn't exist.
  3. Copy data pages to the same address in each database file as they were when the backup was produced. And of course also copy log records to ldf file. Source for these copy operations is the backup file that you restore from.
  4. REDO. Roll forward transactions based on the log records copied in above step.
  5. UNDO. Rollback all open transaction. Not performed if you specify NORECOVERY (more later on STANDBY option). Database is now accessible, and no further backups can be restored (diff or log backups).
A couple of special cases:
 
If you are in bulk-logged recovery model, then a log backup performed if you did minimally logged operations since last log backup will contain also data pages (in addition to log records). This type of backup cannot be performed if any of the data files are lost. When you restore from this type of backup, you cannot do point-in-time restore.
 
The STANDBY option of the RESTORE command does perform UNDO but saves that work to a file you specify. This so that the UNDO work can be undone when you restore a subsequent backup.
 
I think I managed to compress the topic pretty well, so unless you worked a bit with backup and restore in SQL Server, you might want to read above a couple of times. :-)
Published Saturday, October 25, 2008 12:26 PM by TiborKaraszi
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