Backups are useful primarily for two purposes. The first is to restore a state following a disaster (called disaster recovery). The second is to restore small numbers of files after they have been accidentally deleted or corrupted. Data loss is also very common. 66% of internet users have suffered from serious data loss.

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Since a backup system contains at least one copy of all data worth saving, the data storage requirements are considerable. Organizing this storage space and managing the backup process is a complicated undertaking. A data repository model can be used to provide structure to the storage. In the modern era of computing there are many different types of data storage devices that are useful for making backups. There are also many different ways in which these devices can be arranged to provide geographic redundancy, data security, and portability.

Before data is sent to its storage location, it is selected, extracted, and manipulated. Many different techniques have been developed to optimize the backup procedure. These include optimizations for dealing with open files and live data sources as well as compression, encryption, and de-duplication, among others. Many organizations and individuals try to have confidence that the process is working as expected and work to define measurements and validation techniques. It is also important to recognize the limitations and human factors involved in any backup scheme.

Source: http://en.wikipedia.org/wiki/Backup

Backup Methodologies:

Full Back-up – every file

Incremental

• Only files that have been changed or added recently
• Only files with their archive bit set are backed up
• This method is fast and uses less tape space, but has some inherent vulnerabilities, one being that all incremental backups need to be available and restored from the date of the last full backup to the desired date should a restore be needed
• Restore = last full backup plus each incremental

Differential

• Only files that have changed since the last backup
• All files to the full back-up (additive)
• Restore = full backup plus last differential

Types of Tape

• DAT – Digital Audio Tape
• QIC – Quarter Inch Cartridge – small and slow
• 8mm Tape – Superceded by DLT
• DLT – Digital Linear Tape – 4mm tape – large and fast

Other Media

CD – permanent backups, onger shelf life than tape

ZIP or JAZZ – no longer as common

Tape Array – 32 to 63 Tape Array using RAID Technology

HSM – hierarchical; provides a continuous on-line back-up by using optical or tape “jukeboxes”, similar to WORMs