Most backup software provides options for “Full”, “Incremental”, and “Differential” backups. There are a few other variations and terms that you should understand when deciding how to backup your data. The backup utility that is included with the Windows XP operating system uses the terms; “Normal”, “Copy”, “Incremental”, “Differential”, and “Daily”.
Full or Normal – Backup all of the files that are selected regardless of when they were last backed up or when they were last updated. This is the typical backup that requires the most media resources.
Copy – Same as Full, except that the files are not marked as backed up. This is only important when Incremental or Differential backups are in use.
Incremental – Backup only files which are new or have been updated and have not been backed up already. This option requires less media and less time because files that have already been backed up are not backed up again. Use of this option requires careful management and cataloging of your media. In the event that you lose your data, you will need to locate your most recent full backup AND all of the incremental backups that were made since you’re most recent Full backup. For example: if you backup your entire hard-drive every Saturday, and do Incremental backups Sunday thru Friday. Under this plan, if your hard-drive fails on Thursday, you will have to find the tapes that contain last Saturday’s Full backup and Incremental backups made on Sunday, Monday, Tuesday, Wednesday, and Thursday if one had been made before the failure. Once all of the media have been located, they must be restored in the correct order.
Differential – Backup only files which are new or have been updated since the last Full backup. This option is similar to the Incremental type of backup except that each Differential backup contains ALL of the files that have changed since the last Full backup, regardless of whether or not the files were backed up in a previous differential backup. Differential backups are not as fast as Incremental backups and will require may require more media than Incremental backups because if a file changes on Monday, it will be backed up again everyday until the next Full backup. Differential backups also require careful media management because in the event of a data loss, you will have to locate the most recent Full backup and the most recent Differential backup. Differential backups have a distinct advantage over Incremental backups at restore time. At most, two sets of media will be needed to recover data; the Full backup media, and the Incremental backup media.
Daily – Only backup files which are new or changed within the last day. This is very similar to Incremental backups. Daily backups should only be used in a plan that includes periodic Full backups. In the event of a data loss, the most recent Full backup and all Daily backups since the most recent Full backup will be required.
Archive – This term is used when files are moved to the backup media and then deleted from the disk storage.
Daily, Incremental and Differential backups should be used with caution. Most backup systems that were designed for consumers offer these types of backups but don’t provide the media management that is required to recover from a disaster. The more sophisticated packages that do keep track of your media in a catalog do not relieve you of the responsibility to label your tapes or media appropriately and keep your physical media organized. Data centers with IT professionals use Incremental and Differential backups to reduce the time and media requirements, but they also follow rigid processes for labeling and tracking media, including off-site rotation. Daily backups, as offered, by the NT Backup utility are not generally used by IT professionals.
Modern online backup systems make heavy use of Incremental backups. The media management issues do not apply to online backup systems because the media is all handled by the online backup service provider. With online backup systems you get all of the advantages of speed and space savings of Incremental backup without the extra effort required to restore them. Some online backup systems take Incremental backup to a new level by only backing up parts of files that have changed.
Unless you are using an online backup system or you are diligent about managing your media, you are advised to use full backups for your important data. Otherwise, you may find yourself scrambling to find the correct media when you lose your data. Full backups are slower and take more media, however, when you need to restore your files all you have to find is the most recent backup.
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