Full, Incremental, and Differential Backups

A CompTIA Network+ Certification Exam Tutorial

By Chris Bryant, CCIE #12933

I'm an optimist by nature, but not when it comes to my network's data. I work under the assumption that a drive on any server in my network could fail at any moment. I've got a total backup plan in place to replace data lost in any such situation. (And if saying that out loud isn't asking for a drive to fail, well....)

My point is that for you to pass the Network+ exam and be a world-class network administrator, you had better have a solid backup plan in place. And you better be implementing it. Are all of your backups up to date? How are they labeled? How are they stored?

You better have answers to these questions before data is lost, because you don't want to be explaining your lack of answer after a drive failure.

I've rarely seen a network that had no data backup system in place; the question was whether the system was correctly and consistently applied. There's nothing fun about running data backups, and it's the kind of job that tends to be passed around from person to person. ("Where are the backups? It's not my job, it's....well, it's somebody else's!")

Once the responsibility of running backups has been assigned, it's time for the good part. You've got to decide what kind of backups to run, and how often. This decision is crucial, and the correct choice for your network really hinges on how much data you need to back up and how long you've got to do it.

Every file has an archive bit, and that bit is examined every time a backup is run. After a file is backed up, the bit is turned off. When the file's contents are changed, or when a file is first created, the archive bit is turned on. The archive bit has quite a bit to do with our three backup options:

• A full backup is just that - a full backup. Whether the archive bit is on or off, all files will be backed up.

• An incremental backup will back up any file that has been altered or created since the last backup of any kind. That means that all files with the archive bit turned on will be backed up; if the archive bit is off, the file will not be backed up.

• A differential backup will back up all files that have been altered or created since the last full backup only.

It's common to run full backups on weekends only, since a full backup is very time-consuming (depending on the amount of data, of course). Differential or incremental backups can be run during the week.

The backup method you choose directly impacts the number of tapes you'll need for a data restore. Let's say you use the following backup schedule:

Saturday - Full backup

Monday - Incremental backup

Thursday - Incremental backup

If a drive goes bad on Friday, you'll need your full backup from Sunday and both incremental backups to perform a full restore. If differential backups had been used, the last full backup and only the last differential backup tape would have been needed.

Whether you choose incremental or differential backups to compliment the full backup is up to you. What is truly important is that the backups are run, verified every morning after the backup, and that the storage media is clearly labeled with the date of the backup.

Knowing the theory of these backup methods is important for the CompTIA Network+ exam, but the key to success with backups in the real world is to run them.

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To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com