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.
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