In describing incremental backup schemes, three major types of backups come into play:
Full Backup A full backup contains all the files on a system, or at least all the files that are important. (Full backups may deliberately omit the contents of temporary directories such as /tmp, for instance.) If you perform only full backups, you can restore anything or everything from the system using just one backup medium or set of media; but such backups take the most time and consume the most media space.
Incremental Backup An incremental backup contains only those files that have been added or changed since the last full backup. (As with full backups, you may deliberately omit certain files or types of files.) If you perform a full backup on Monday and incremental backups every other day of the week, the incremental backups will gradually increase in size over the week. Restoring data requires using two media or sets of media: the full backup and the latest incremental backup.
Differential Backup A differential backup contains only those files that have been added or changed since the last backup of any type. (Again, you can omit specific files or file types, if you like.) If you perform a full backup on Monday and differential backups every other day of the week, the differential backups are likely to vary randomly in size. Restoring data may require accessing the full backup and every differential backup made since the full backup.
Each backup type represents a different trade-off between backup convenience and restore convenience. All backup schedules include at least periodic full backups. Including incremental or differential backups (or both) in a backup schedule is usually beneficial. Precisely how often to perform each backup type is an issue that must be decided on a case-by-case basis, as described in the next section, "Creating a Backup Schedule."
Warning Incremental and differential backups store new files, but most backup programs don't record the fact that deleted files have been deleted. Therefore, a full restore from a complete set of backups may include more files than were stored on the system at any given point in time. (A very new backup program, Duplicity, http://www.nongnu.org/duplicity/, claims to record information on deleted files.) If disk space is tight, or if your users regularly create and delete very large files, be sure to consider this fact when designing a backup plan, lest you run out of disk space when you restore data.
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