What could be more frustrating than knowing that your database backups went missing? This is specifically true if you are dealing with transaction log backups which are dependent on log sequence numbers. You don’t want to lose a single transaction log backup in the chain. In previous versions of SQL Server, we just execute a copy command (or even ROBOCOPY) to copy the transaction log backups to a different location. In SQL Server 2005, we have the MIRROR TO clause. This specifies a set of one or more backup devices that will mirror the backups devices specified in the TO clause, which could be a tape, disk or network location.

To use the MIRROR TO clause, see the sample script below

BACKUP DATABASE AdventureWorks
TO DISK = 'DAdventureWorksDB2.BAK'
MIRROR to DISK = 'F:\AdventureWorksDB.BAK'
WITH INIT, FORMAT, STATS=10
GO

This creates a mirrored copy of the database backup of the AdventureWorks database to the F: drive. Although this creates a mirrored copy of the backup, it will definitely take quite some time to complete as it is writing on all the mirrored media set, thereby, increasing database your backup window. I did a test on this by using the MIRROR TO DISK clause with the AdventureWorks database and it takes like 10 times longer to do a backup with this option – the speed, of course, would be dependent on where you mirror your backups and its IO performance

I’d probably still stick to using the copy command in my backups as long as I get the same result. For more information on using mirrored backup media sets, check out this MSDN article