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Two methods for differential backups

I have three 20-GB tapes drive and 18 GB of data. The first day of each week, I do a full backup on the first tape. The second day, I make a differential backup on the second tape. Before I do that, I need to enter the first tape. Now, prior to making a third differential backup tape, I need to enter the first and second tape. Do you have a solution to my problem?

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Without knowing the software you're using, it would be very difficult to provide you with the steps needed to set up your application to provide this solution for you. However, you should not need the tape containing the full backup each time you perform a differential backup. Most of the backup programs allow you to perform a backup of "changed" files. The way that it is determined if the file has changed is based upon the archive attribute assigned to the file while it resides on your hard disk. Now, you have a couple of options on how you handle backing data up this way. Both involve performing a full backup (we'll call this the baseline backup). You would configure the backup application to reset the archive attribute to normal. This means that all of the files have been backed up. From this point on, any file that is opened and edited or changed in any way will have this attribute set as archive.

The first method for the differential backup would be to do the same as above, but only backing up the files that have the archive attribute set. Again, upon completing the backup, all the attributes would be changed back to normal.

The second method would be to back up just the files that have the archive attribute set, but in this scenario, you would leave the attribute set, instead of changing it back to normal.

The difference between the two methods above is that the first method will only back up files that have changed since the last backup. The second method will back up all the files that have changed since the last baseline backup.

The first method will typically take less time to perform each backup, since the number of files changed is likely to be less, since the attribute is constantly being reset. But the downside to this method is that unless you frequently do baseline backups, you may lose files that had changed a several backups ago, and haven't been backed up since.

The benefit of the second method is that if the worst should occur, you would really only need to restore two tapes to get you back to where you were prior to the crash -- your last baseline backup and your last differential backup. The reason for this is that your differential backup will contain ALL the files (in their most recent version) since your last baseline backup. The downside to this method is that the amount of changed data being backed up is likely to increase over the course of time and will only get reset when you perform your next baseline backup.

This was first published in February 2005

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