B Esakkiappan's SQL Thoughts

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Know The Transaction Log – Part 4 - Restoring Data

After looking about the backups in SQL Server it is time to know about the Restore and Recovery in SQL Server.

SQL Server supports three levels of Restoring data. They are

1. Complete Database Restore : This is the basic restore strategy. A complete database restore for a simple recovery model database simply involve a full backup followed by the latest differential backup if any available. For a full or bulk-logged recovery model database this complete Data Restore involves restoring a Full Backup followed by a latest differential backup and then a sequence of Transaction log backup in which they are backed up and finally tail-log backup if any available.

2. File Restore : This restore operation is very useful when any one of the files in the file group is damage. The main advantage of this restore is restore time will be less, obviously compared to complete database restore. For simple recovery model database file restore will work with read-only secondary files.

3. Page Restore : This restore is only applicable for Full or Bulk-Logged Recovery model database and not available for Simple recovery model. Using this level of restore , a particular page or pages can be restored.

How Restore Works ?

Restoring is the process of copying data from a backup and applying Transaction logs to the data to point of time when the backup is taken. This process is done in three phases, Data Copy phase, Redo Phase and Undo Phase.

Data Copy Phase: The process of copying the data, index and log pages from the backup media to the database files. No log backups nor log information in the data backups are applied in this phase.

Redo Phase : This phase applies logged changes to the data by processing log information from the log backups.

Undo Phase : This phase applies undoing any uncommitted transactions from the data that are restored from Backup and brings the database online.

In this stage we have to understand the relationship between the WITH RECOVERY and WITH NORECOVERY options in RESTORE Command.

The default option is WITH RECOVERY. This will continue the Undo phase after completing the REDO phase.

A normal restore operation stops at the redo stage if WITH NORECOVERY is included in RESTORE statement. This allows Roll Forward to continue with the next statement of the Restore Sequence, in which the other backups  typically a differential or a transaction backup will do the undo phase.

For a Full Recovery model database or for a Bulk-logged recovery model database, a restore operation is done by a sequence of RESTORE statements. This sequence is called Restore Sequence.

For a simple scenario a restore sequence might be

· starting with restoring a recent full backup,

· applying the most recent differential backup,

· restoring the sequence of log backups in the order they are backed up after the most recent differential backup,

· finally the restoring tail log backup if any taken after the failure occurred.

For more complex scenarios, complex sequence planning will be required. For these planning a recovery path is very important. A Recovery Path is a complete sequence of data and log backups that can  bring database to a point of time. For more details about Recovery Path search in Books On Line.

Complete Database Restore :

A simple restore strategy. Let us see how you have to do a Complete Database Restore using an example. Suppose for a full or bulk-logged recovery model database, a series of backups are taken in the following schedules. A Full backup on Monday 10 PM, Differential backups are scheduled on 10 PM of Wednesday,Friday and Sunday. Transaction Log Backups are scheduled twice a day 6 AM and 6 PM every day. In this sequence, the database is got failed on Saturday 4 PM. How to Restore this database with available backups ?

On a failure situation of database, first thing we have to do is take the tail log backup with NOTRUNCATE option if possible. So take the Tail-log backup first.

Every Restore is to be started with Full Backup. So start with restoring the full backup taken on Monday 10 PM with NORECOVERY option. We have the latest differential backup taken on Friday 10 PM. So apply that backup, and we can omit applying log backups taken after Monday 10 PM and before Friday 10 PM. After restoring this latest differential backup, we have to restore the log backup taken on Saturday 6 AM. That is latest log backup taken on schedule before failure. Now the database is ready up to the Saturday 6 AM. Now, Restore the tail backup that is taken after the failure , that Saturday 4 PM with RECOVERY option. Now the database is fully restored.

File or File Group Restore :

To Restore a Single File in a File group or Complete File group of a Database you have to use Restore Command with FILE option or FILEGROUP option. All you need is unbroken chain of log backups from the time of file or file group backup was made. Before applying the file  or file group backup you have to take the transaction log backup. After restoring the file or file group , you have to restore all the transaction log backups to synchronise that file or file group with the rest of the database.

Let us see an example. Suppose a SecondaryFG is a file group of a database backed up on Friday 12 noon and the database is still in use. Backing up of Transaction log of this database is scheduled on 10 AM, 11:30 AM, 1:00 PM, 2:30 PM , 4:00PM, 5:30PM and so on. Note that the database is still in use, and the changes are made in SecondaryFG and other file groups too. At 5:15 PM, a media failure occurs that corrupts the SecondaryFG. Now we have to restore this. First take the tail log backup that contains all the log records after 4:00 PM Log Backup, with NOTRUNCATE NORECOVERY, to make the database in Restoring state so that no other modification will be done after the failure. Now apply the backup that was taken at 12 noon. So the SecondaryFG now have  all the changes that are made up to 12 noon. Now start applying the Transaction log backups in the sequence of they backed up that is apply 1:00PM Backup first, 2:30 PM backup second, 4:00 PM backup third. Now the SecondaryFG is synchronised with all the database files up to 4:00 PM. Finally apply the tail log backup that was taken after the failure to make SecondaryFG fully compatible with all the files of the database.

Page Restore:

Page restore is only possible for databases using Full Recovery model or Bulk-logged Recovery model. All Editions of SQL Sever other than Enterprise Edition  support offline Page Restore whereas SQL Server 2005 Enterprise Edition supports Online PAGE Restore when database is online.

A page may be marked as suspect page, when a query or DBCC CHECK TABLE or DBCC CHECKDB  failed to access it. Every page in a database that is marked as suspect will have an entry in msdb..suspect_pages table. Event_type column of this table may have either one of the following numbers, 1 for the pages marked with error number 824 other than Bad Page ID and Checksum Error; 2 for Bad PageID ,3 for Checksum Error. 4,5 and 7 for the repaired pages. This table is limited to size and if it is full, the errors could not be logged in this table. So it should be a DBA’s routine to delete the all the records in the msdb..suspect_pages table having event_type is greater than or equal to 4 in regular intervals.

Get the pageId and fileId from msdb..suspect_pages for the pages to be restored. Start RESTORE with full or File or Group Backup that contains the pages to be restored and use PAGE option. Then apply the most recent differential backup if any available and apply all subsequent log backups. Now backup the log and restore it again to match the last_target_LSN in sys.masterfile.

Limitations of Page Restore

· Only Database pages can be restored not the log pages.

· File boot page i.e Page 0 can not be restored and page 1:9 can not be restored that is database boot page.

· GAM , SGAM and PFS Pages can not be restored.

For more details see Books Online.

With this I conclude my Know The Transaction LOG Series. Some things are  purposely omitted in this series of post that are POINT-IN-TIME Restore using RESTORE with STOPAT option to avoid over doses.

Note: This post was originally published in my  SQLThoughts blog.


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# June 26, 2008 7:07 AM