Backup

In information technology, a backup is a copy of computer data stored in a separate location. This process allows users to restore original files after a data loss event occurs. Data loss can happen through accidental deletion or through data corruption. Backups serve as a fundamental method for IT disaster recovery. However, not all backup systems are powerful enough to reconstitute complex configurations. These complex systems might include a computer cluster, a database server, or an active directory server. A complete backup system must contain at least one copy of every piece of data deemed worth saving.

To manage these copies, organizations use an information repository. This is a secondary storage space that gathers all the various data sources. A repository can be very simple, such as a list of all backup media and their creation dates. More advanced repositories use a computerized index, a catalog, or a relational database to track information. The process of creating a backup involves selecting, extracting, and manipulating data for storage. This often includes technical methods like compression to save space or encryption to keep data secure. Some systems even use de-duplication to avoid saving the same data twice. To ensure reliability, many schemes include dry runs to validate that the data is actually usable.

Experts often follow the 3-2-1 backup rule to ensure maximum safety. This rule suggests maintaining at least 3 copies of your data. These copies should be stored on 2 different types of storage media. Finally, at least 1 copy should be kept in an offsite or remote location. Using different media helps prevent loss from specific technical failures. For example, optical discs might survive being underwater, while LTO tapes might not. Similarly, solid-state drives (SSDs) do not have moving parts, so they cannot fail due to head crashes like hard drives. An offsite copy, which can include cloud storage, protects against physical threats like fire, theft, or natural disasters like earthquakes and floods.

There are several distinct methods for performing backups. A full backup creates a complete copy of all source data at a specific point in time. This is often used to create system images, which record known good configurations for computer technicians. An incremental backup is different because it only stores data that has changed since a specific reference point. To restore data from an incremental system, you must start with the last full backup and then apply every incremental backup in order. A differential backup also saves changed data, but it saves everything changed since the last full backup. This means you only need two backups—the last full one and the most recent differential one—to perform a restore.

More advanced methods provide even tighter protection. Continuous Data Protection, or CDP, instantly saves a copy of every single change made to the data. This allows a user to restore data to almost any specific point in time. Many commercial applications use "Near-CDP," which takes incremental backups at set intervals, such as every 15 minutes or every 24 hours. These systems often use journaling and snapshots, which are read-only copies of data frozen at a specific moment. Another method is the reverse incremental backup. This method maintains a recent mirror of the source data and stores the differences between the current mirror and previous states. This can be done using binary diffs or hard links, a method used by Apple Time Machine.

Different types of physical media are used as backup destinations. Magnetic tape was once the most common medium for bulk data storage. It is a sequential access medium, meaning it is very fast at reading or writing data in a continuous stream. While the media itself is inexpensive, the tape drives can be dozens of times more expensive than hard disk drives. By 2014, LTO had become the primary technology for tape. Hard disks are also widely used because they are cheap and provide quick access to data. Some modern portable hard drives use accelerometer technology, or shock sensors, to help them survive being dropped.

Optical storage, such as CDs, DVDs, and Blu-ray Discs, uses lasers to read and retrieve data. These discs are generally cheaper and have lower capacities than hard disks or tapes. One major advantage of optical storage is that it is not vulnerable to water. This makes optical discs a strong choice for surviving a flood disaster. However, users must watch for media degradation over time. This can be predicted by measuring the rate of correctable minor data errors. If these errors increase consecutively, the risk of uncorrectable sectors grows. Understanding these various tools and methods helps ensure that digital information remains safe and recoverable.