SQL SERVER – BACKUP DATABASE ,Mirrored Backup and Restore and Split File Backup

Introduction

This article is based on a real life experience of the author while working with database backup and restore during his consultancy work for various organizations. We will go over the following important concepts of database backup and restore.

1. Conventional Backup and Restore
2. Spilt File Backup and Restore
3. Mirror File Backup
4. Understanding FORMAT Clause
5. Miscellaneous details about Backup and Restore

Note: Before running all the examples, make sure that you have the required folders created on your drive. It is mandatory to create Backup folders prior to creating backup files using SQL Server.

In our example, we will require the following folders:

• C:\Backup\SingleFile
• C:\Backup\MultiFile
• C:\Backup\MirrorFile

Conventional and Split File Backup and Restore

Just a day before working on one of the projects, I had to take a backup of one database of 14 GB. My hard drive lacked sufficient space at that moment. Fortunately, I had two 8 GB USB Drives with me. Now, the question was how to take a backup in two equal sizes, each of 7 GB, so I can fit them on each USB drive. Well, conventional backup takes one large backup in one file. However, SQL Server backup command can take backups in two or more split parts.

Let us see an example of a conventional one-file backup using the AdventureWorks database.

BACKUP DATABASE AdventureWorksTO DISK = 'C:\Backup\SingleFile\AdventureWorks.bak'GO

The result is displayed below. Here, the backup is taken in a single file.

Now, let us see how we can split one database into two different database files. This method is very similar to taking a single-file backup. By simply adding an additional DISK option we can split the files backup files.

BACKUP DATABASE AdventureWorksTO DISK = 'C:\Backup\MultiFile\AdventureWorks1.bak',DISK = 'C:\Backup\MultiFile\AdventureWorks2.bak',DISK = 'C:\Backup\MultiFile\AdventureWorks3.bak'GO

In the previous example, we can clearly see that backup is split into three equal parts of the original backup file size.

Restoring a backup from a single-file backup is quite easy. Let us go over an example where we restore the AdventureWorks database from a single backup file.

RESTORE DATABASE AdventureWorksFROM DISK = 'C:\Backup\SingleFile\AdventureWorks.bak'GO

Running the above script will give a successful message.

Now let us see an example where we restore a database from a split file. This method is very similar to restoring a database from a single file; just add an additional DISK option.

RESTORE DATABASE [AdventureWorks]FROM DISK = N'C:\Backup\MultiFile\AdventureWorks1.bak',DISK = N'C:\Backup\MultiFile\AdventureWorks2.bak',DISK = N'C:\Backup\MultiFile\AdventureWorks3.bak'GO

Running the above script will give a successful message as shown in the image below.

Make sure that while restoring database, the database is not in use, otherwise it will give an error of database in use. In the event of an error taking place, close all the connections and re-attempt to restore the database.

Mirror Backup of the file

It is quite a common practice to create an exact copy of the backup and store it to several places to deal with any catastrophes which might affect the place where the database is stored. Once a full backup is accomplished DBAs generally copy the database to another location in their network using a third party tools like robocopy or native DOS commands like xcopy.

In SQL Server 2005 and later versions, there is a Mirror command that makes a copy of the database backup to different locations while taking the original backup. The maximum limit of additional locations that can be specified with MIRROR clause is 3.

Mirrored backup can be taken in local computer system as well as in a local network. Let us now see two examples of mirror backup.

Example 1. Single File Backup to Multiple Locations using Mirror

BACKUP DATABASE AdventureWorksTO DISK = 'C:\Backup\SingleFile\AdventureWorks.bak'MIRROR TO DISK = 'C:\Backup\MirrorFile\AdventureWorks.bak'WITH FORMAT
GO

If this command is being run for the first time, it is mandatory to use the WITH FORMAT clause; but for sub sequential runs it is not required. WITH FORMATreinitializes the backup.

When checked in both the folders ‘SingleFile’ and ‘MirrorFile’, backup files are exactly the same files. As mentioned earlier, four mirror backup can be specified in total.

Example 2. Split File Backup to Multiple Locations using Mirror

We have earlier seen an example where we can have multiple split files of large database backup files. SQL Server Mirror functionality also supports backup of the split files.

BACKUP DATABASE AdventureWorksTO DISK = 'C:\Backup\MultiFile\AdventureWorks1.bak',DISK = 'C:\Backup\MultiFile\AdventureWorks2.bak',DISK = 'C:\Backup\MultiFile\AdventureWorks3.bak'MIRROR TO DISK = 'C:\Backup\MirrorFile\AdventureWorks1.bak',DISK = 'C:\Backup\MirrorFile\AdventureWorks2.bak',DISK = 'C:\Backup\MirrorFile\AdventureWorks3.bak'WITH FORMAT
GO

All the mirror sets will need the same number of DISK clauses as the original backup media.

Mirrored database backup can be restored using the same method as the original backup. Mirrored backup is in fact an exact replica of the original backup.

Understanding the FORMAT Clause

The FORMAT clause is used to reinitiate a backup media. Although it is a very useful clause it should be used with caution. When the clause is used it erases everything present in backup media. I have noticed that some DBAs are confused while taking a backup on a local disk where they have SQL Server installed. They have a misconception that if the format command is used, it will erase the complete disk including the SQL Server installation. However, the fact is that SQL Server format clause is quite different from OS format. The effect of SQL Server format clause is limited to a folder or path specified in the DISKclause.

In our example, when the FORMAT clause is specified, it will format only folders like C:\Backup\MultiFile\ or C:\Backup\SingleFile.