When it’s time to design a backup schedule, you can use three key types of backups: full, differential, and incremental.
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 considerably slow down the network. In addition, some environments have more data than can fit on a single tape. This makes doing a full backup awkward because someone might need to be there to 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. If a failure occurs, 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 could be restored on another computer. Using passwords on tape backups and using a secure offsite and onsite location can minimize the security risk.
Companies that don’t have enough time to complete a full backup daily can use the differential backup. Differential backups are faster than a full backup because 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 requires the last full backup and the latest differential backup.
Differential backups know what files have changed since the last full backup because they use a setting called the archive bit. The archive bit flags files that have changed or have been created and identifies them as ones that need to be backed up. Full backups do not concern themselves with the archive bit because all files are backed up, regardless of date. A full backup, however, does clear the archive bit after data has been backed up to avoid future confusion. Differential backups notice the archive bit and use it to determine which files have changed. The differential backup does not reset the archive bit information.
Some companies have a finite amount of time they can allocate to backup procedures. Such organizations are likely to use incremental backups in their backup strategy. Incremental backups save only the files that have changed since the last full or incremental backup. Like differential backups, incremental backups use the archive bit to determine which files have changed since the last full or incremental backup. Unlike differentials, however, incremental backups clear the archive bit, so files that have not changed are not backed up.
The faster backup time of incremental backups comes at a price—the amount of time required to restore. Recovering from a failure with incremental backups requires numerous tapes—all the incremental tapes and the most recent full backup. For example, if you have a full backup from Sunday and an incremental for Monday, Tuesday, and Wednesday, you need four tapes to restore the data. Each tape in the rotation is an additional step in the restore process and an additional failure point. One damaged incremental tape, and you cannot restore the data.
Published on Fri 14 October 2011 by Manny Larson in Computer Science with tag(s): backups