Backup
Backup in computer engineering refers to the copying of data for the purpose of having a second copy of an original source, in case of damage to the original data source. The "data" in question may be either data as such, or stored program code, both of which are treated the same by the backup software.
The word may be used as a noun, e.g., "have you remembered to move the backup to a safe place?", or as a verb, "he didn't back up the data, so we lost last week's work". Also common are various combinations, such as backup copy, backup software (the applications that are used for performing the backing up of data, i.e., the systematic generation of backup copies), backup policy (an organisation's procedures and rules for ensuring that adequate amounts and types of backups are made, including suitably frequent testing of the process for restoring the original (production) system from the backup copies).
As of 2004, backups are most often made from hard disk based production systems to large capacity magnetic tape storage, or optical disk WORM media like CD-R and DVD-R and similar formats. During the period 1975–95, most personal/home computer users associated backup mostly with copying floppy disks.
There are four primary metrics relating to data backup:
- Recovery Point Objective (RPO) is the point in time that the restarted infrastructure will reflect. Essentially, this is the roll-back that will be experienced as a result of the recovery. Reducing RPO requires increasing synchronicity or frequency of copying the data to be protected.
- Backup Window is the amount of time that taken to copy a given data set to the backup device. Most traditional backup systems require a data set to be frozen for hours while the entire content of a filesystem is copied to magnetic tape. Newer techniques use a mirror or snapshot of a set of data as the backup source, effectively reducing the required backup window.
- Restore Time is the amount of time required to bring a desired data set back from the backup media.
- Retention Time is the amount of time in which a given set of data will remain available for restore. Some backup products rely on daily copies of data and measure retention in terms of days. Others retain a number of copies of data changes regardless of the amount of time.
Computer backups are useful primarily for two purposes, the first and most obvious is to restore a computer to an operational state following a disaster, such as loss of a hard disc or the file system becoming so badly corrupted it cannot be read. The second use, often overlooked but probably more common, is to facilitate the recovery of a single file or set of files when they are accidentally deleted or corrupted by the user or a program.
The location at which the backup is stored is also of high relevance. In particular, storing the copy near the original is unwise, since some disasters such as fire is likely to cause the backup the same damage as the original. Storing backups off-site is an important part of a backup policy.
A backup is only as useful as its associated recovery strategy. Having a complete set of backup tapes is of no use if the only copy of the software required to read them is on one of the tapes. It is also possible for backup software to run successfully for several months, only to fail when it is needed most due to read errors on the backup media. Magnetic tapes in particular should be read-tested on a regular basis.
