Full Backup

The preferred method of backup is the full backup method, which copies all files and directories from the hard disk to the backup media. There are a few reasons why doing a full backup is not always possible. First among them is likely the time involved in performing a full backup.

Depending on the amount of data to be backed up, full backups can take an extremely long time and can use extensive system resources. Depending on the configuration of the backup hardware, this can slow down the network considerably. In addition, some environments have more data than can fit on a single tape. This makes taking a full backup awkward, as someone may need to be there to manually change the tapes.

The main advantage of full backups is that a single tape or tape set holds all the data you need backed up. In the event of a failure, a single tape might be all that is needed to get all data and system information back. The upshot of all this is that any disruption to the network is greatly reduced.

Unfortunately, its strength can also be its weakness. A single tape holding an organization's data can be a security risk. If the tape were to fall into the wrong hands, all the data can be restored on another computer. Using passwords on tape backups and using a secure offsite and onsite location can minimize the security risk.

Differential Backup

For those companies that just don't quite have enough time to complete a full backup daily, there is the differential backup. Differential backups are faster than a full backup, as they back up only the data that has changed since the last full backup. This means that if you do a full backup on a Saturday and a differential backup on the following Wednesday, only the data that has changed since Saturday is backed up. Restoring the differential backup will require the last full backup and the latest differential backup.

Differential backups know what files have changed since the last full backup by using a setting known as the archive bit. The archive bit flags files that have changed or been created and identifies them as ones that need to be backed up. Full backups do not concern themselves with the archive bit, as all files are backed up regardless of date. A full backup, however, will clear the archive bit after data has been backed up to avoid future confusion. Differential backups take notice of the archive bit and use it to determine which files have changed. The differential backup does not reset the archive bit information.