Backup

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All computer storage systems are inherently volatile, and subject to electrical and mechanical damage, as well as deteriorating with age.

A backup is a separate copy of a set of files held on different media from the media primarily used, and only accessed for the purpose of restoring a damaged or deleted file. A backup is not an archive, which attempts to preserve files with permanence, but does not typically include the computer's operating system or file hierarchies.

Backups are themselves just as likely to be damaged as the primary media, so a good backup routine limits risk rather than removing it.

There are some key concepts in the practice of backing up.

• An offsite backup is where the media is physically removed from the premises, dramatically decreasing the chances that an event such as a fire or electrical failure will damage both the primary media and the backup.
• A full backup is when the entire contents of the data system, typically the hard disk of the computer, are copied onto different media
• An incremental backup is a regular, often daily or hourly, backup of all the files which have changed since the last full backup
• Grandfather, father, son refers to a backup regime where the media is overwritten in three generations. For example, a full backup is made onto tape on Week 1, a second backup onto a separate tape in Week 2, a third backup on Week 3. In Week 4, a new full backup is made on the original tape, overwriting the Week 1 backup.
  • It should be clear that errors which have entered the system can only be recovered with this strategy if they are found within three weeks.
  • This approach is not suitable for incremental backup, because it would mean that only files which had been altered since the grandfather backup would be protected
• A snapshot is a representation of the way all the files were at a particular moment. A full backup is by definition a snapshot, but some systems, for example, Apple's Time Machine is able to present a snapshot based on a combination of full and incremental backups.
• An automated backup is a system which can function without human intervention, and, typically, does not require the user to change media such as backup tapes. Automated backup is typically to a file server, either on the local area network or on a wide area network such as the internet.

Even the most rudimentary backup routine requires twice the amount of storage as the total size of the files to be backed up, as this allows for a full backup to be made without first erasing the previous full backup. A grandfather, father, son backup requires three times the amount of space, and, if linked to an incremental backup as well, probably requires a total of four times the amount of storage.