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Stored data in computer systems
duplicate data in computer systems for data recovery

In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", whereas the noun and adjective form is "backup". Backups can be used to recover data after its loss from data deletion or corruption, or to recover data from an earlier time. Backups provide a simple form of disaster recovery; however not all backup systems are able to reconstitute a computer system or other complex configuration such as a computer cluster, active directory server, or database server. A backup system contains at least one copy of all data considered worth saving. The data storage requirements can be large. An information repository model may be used to provide structure to this storage. There are different types of data storage devices used for copying backups of data that is already in secondary storage onto archive files. There are also different ways these devices can be arranged to provide geographic dispersion, data security, and portability. Data is selected, extracted, and manipulated for storage. The process can include methods for dealing with live data, including open files, as well as compression, encryption, and de duplication. Additional techniques apply to enterprise client server backup. Backup schemes may include dry runs that validate the reliability of the data being backed up. There are limitations and human factors involved in any backup scheme.

A backup strategy requires an information repository, "a secondary storage space for data" that aggregates backups of data "sources". The repository could be as simple as a list of all backup media (DVDs, etc.) and the dates produced, or could include a computerized index, catalog, or relational database. The backup data needs to be stored, requiring a backup rotation scheme, is generally more useful as a way of deploying a standard configuration to many systems rather than as a tool for making ongoing backups of diverse systems.

An incremental backup stores data changed since a reference point in time. Duplicate copies of unchanged data are not copied. Typically a full backup of all files is made once or at infrequent intervals, serving as the reference point for an incremental repository. Subsequently, a number of incremental backups are made after successive time periods. Restores begin with the last full backup and then apply the incrementals. Some backup systems can create a synthetic full backup from a series of incrementals, thus providing the equivalent of frequently doing a full backup. When done to modify a single archive file, this speeds restores of recent versions of files.

Continuous Data Protection (CDP) refers to a backup that instantly saves a copy of every change made to the data. This allows restoration of data to any point in time and is the most comprehensive and advanced data protection. Near CDP backup applications—often marketed as "CDP"—automatically take incremental backups at a specific interval, for example every 15 minutes, one hour, or 24 hours. They can therefore only allow restores to an interval boundary. read only copies of the data frozen at a particular point in time. Near CDP (except for Apple Time Machine) intent logs every change on the host system, often by saving byte or block level differences rather than file level differences. This backup method differs from simple disk mirroring in that it enables a roll back of the log and thus a restoration of old images of data. Intent logging allows precautions for the consistency of live data, protecting self consistent files but requiring applications "be quiesced and made ready for backup." Near CDP is more practicable for ordinary personal backup applications, as opposed to true CDP, which must be run in conjunction with a virtual machine or equivalent and is therefore generally used in enterprise client server backups. Software may create copies of individual files such as written documents, multimedia projects, or user preferences, to prevent failed write events caused by power outages, operating system crashes, or exhausted disk space, from causing data loss. A common implementation is an appended ". bak" extension to the file name.

A Reverse incremental backup method stores a recent archive file "mirror" of the source data and a series of differences between the "mirror" in its current state and its previous states. A reverse incremental backup method starts with a non image full backup. After the full backup is performed, the system periodically synchronizes the full backup with the live copy, while storing the data necessary to reconstruct older versions. This can either be done using hard links—as Apple Time Machine does, or using binary diffs.

A differential backup saves only the data that has changed since the last full backup. This means a maximum of two backups from the repository are used to restore the data. However, as time from the last full backup (and thus the accumulated changes in data) increases, so does the time to perform the differential backup. Restoring an entire system requires starting from the most recent full backup and then applying just the last differential backup. A differential backup copies files that have been created or changed since the last full backup, regardless of whether any other differential backups have been made since, whereas an incremental backup copies files that have been created or changed since the most recent backup of any type (full or incremental). Changes in files may be detected through a more recent date/time of last modification file attribute, and/or changes in file size. Other variations of incremental backup include multi level incrementals and block level incrementals that compare parts of files instead of just entire files.

Regardless of the repository model that is used, the data has to be copied onto an archive file data storage medium. The medium used is also referred to as the type of backup destination.

Magnetic tape was for a long time the most commonly used medium for bulk data storage, backup, archiving, and interchange. It was previously a less expensive option, but this is no longer the case for smaller amounts of data. Tape is a sequential access medium, so the rate of continuously writing or reading data can be very fast. While tape media itself has a low cost per space, tape drives are typically dozens of times as expensive as hard disk drives and optical drives. Many tape formats have been proprietary or specific to certain markets like mainframes or a particular brand of personal computer. By 2014 LTO had become the primary tape technology. The other remaining viable "super" format is the IBM 3592 (also referred to as the TS11xx series). The Oracle StorageTek T10000 was discontinued in 2016.

The use of hard disk storage has increased over time as it has become progressively cheaper. Hard disks are usually easy to use, widely available, and can be accessed quickly. In the mid 2000s, several drive manufacturers began to produce portable drives employing ramp loading and accelerometer technology (sometimes termed a "shock sensor"), and by 2010 the industry average in drop tests for drives with that technology showed drives remaining intact and working after a 36 inch non operating drop onto industrial carpeting. Some manufacturers also offer 'ruggedized' portable hard drives, which include a shock absorbing case around the hard disk, and claim a range of higher drop specifications. Over a period of years the stability of hard disk backups is shorter than that of tape backups.

Optical storage uses lasers to store and retrieve data. Recordable CDs, DVDs, and Blu ray Discs are commonly used with personal computers and are generally cheap. The capacities and speeds of these discs have typically been lower than hard disks or tapes. Advances in optical media may shrink that gap in the future. Potential future data losses caused by gradual media degradation can be predicted by measuring the rate of correctable minor data errors, of which consecutively too many increase the risk of uncorrectable sectors. Support for error scanning varies among optical drive vendors. Many optical disc formats are WORM type, which makes them useful for archival purposes since the data cannot be changed. Moreover, optical discs are not vulnerable to head crashes, magnetism, imminent water ingress or power surges; and, a fault of the drive typically just halts the spinning. Optical media is modular; the storage controller is not tied to media itself like with hard drives or flash storage (→flash memory controller), allowing it to be removed and accessed through a different drive. However, recordable media may degrade earlier under long term exposure to light. Some optical storage systems allow for cataloged data backups without human contact with the discs, allowing for longer data integrity. A French study in 2008 indicated that the lifespan of typically sold CD Rs was 2–10 years, but one manufacturer later estimated the longevity of its CD Rs with a gold sputtered layer to be as high as 100 years. Sony's proprietary Optical Disc Archive

Solid state drives (SSDs) use integrated circuit assemblies to store data. Flash memory, thumb drives, USB flash drives, CompactFlash, SmartMedia, Memory Sticks, and Secure Digital card devices are relatively expensive for their low capacity, but convenient for backing up relatively low data volumes. A solid state drive does not contain any movable parts, making it less susceptible to physical damage, and can have huge throughput of around 500 Mbit/s up to 6 Gbit/s. Available SSDs have become more capacious and cheaper. Cloud based backup (through services like or similar to Google Drive, and Microsoft OneDrive) provides a layer of data protection.

Online backup storage is typically the most accessible type of data storage, and can begin a restore in milliseconds. An internal hard disk or a disk array (maybe connected to SAN) is an example of an online backup. This type of storage is convenient and speedy, but is vulnerable to being deleted or overwritten, either by accident, by malevolent action, or in the wake of a data deleting virus payload.

Nearline storage is typically less accessible and less expensive than online storage, but still useful for backup data storage. A mechanical device is usually used to move media units from storage into a drive where the data can be read or written. Generally it has safety properties similar to on line storage. An example is a tape library with restore times ranging from seconds to a few minutes.

Off line storage requires some direct action to provide access to the storage media: for example, inserting a tape into a tape drive or plugging in a cable. Because the data is not accessible via any computer except during limited periods in which they are written or read back, they are largely immune to on line backup failure modes. Access time varies depending on whether the media are on site or off site.

Backup media may be sent to an off site vault to protect against a disaster or other site specific problem. The vault can be as simple as a system administrator's home office or as sophisticated as a disaster hardened, temperature controlled, high security bunker with facilities for backup media storage. A data replica can be off site but also on line (e. g., an off site RAID mirror). Such a replica has fairly limited value as a backup.

A backup site or disaster recovery center is used to store data that can enable computer systems and networks to be restored and properly configured in the event of a disaster. Some organisations have their own data recovery centres, while others contract this out to a third party. Due to high costs, backing up is rarely considered the preferred method of moving data to a DR site. A more typical way would be remote disk mirroring, which keeps the DR data as up to date as possible.

A backup operation starts with selecting and extracting coherent units of data. Most data on modern computer systems is stored in discrete units, known as files. These files are organized into filesystems. Deciding what to back up at any given time involves tradeoffs. By backing up too much redundant data, the information repository will fill up too quickly. Backing up an insufficient amount of data can eventually lead to the loss of critical information.

Copying files: Making copies of files is the simplest and most common way to perform a backup. A means to perform this basic function is included in all backup software and all operating systems. Partial file copying: A backup may include only the blocks or bytes within a file that have changed in a given period of time. This can substantially reduce needed storage space, but requires higher sophistication to reconstruct files in a restore situation. Some implementations require integration with the source file system. Deleted files: To prevent the unintentional restoration of files that have been intentionally deleted, a record of the deletion must be kept. Versioning of files: Most backup applications, other than those that do only full only/System imaging, also back up files that have been modified since the last backup. "That way, you can retrieve many different versions of a given file, and if you delete it on your hard disk, you can still find it in your [information repository] archive." Because the disk is read sequentially and with large buffers, this type of backup can be faster than reading every file normally, especially when the filesystem contains many small files, is highly fragmented, or is nearly full. But because this method also reads the free disk blocks that contain no useful data, this method can also be slower than conventional reading, especially when the filesystem is nearly empty. Some filesystems, such as XFS, provide a "dump" utility that reads the disk sequentially for high performance while skipping unused sections. The corresponding restore utility can selectively restore individual files or the entire volume at the operator's choice. Identification of changes: Some filesystems have an archive bit for each file that says it was recently changed. Some backup software looks at the date of the file and compares it with the last backup to determine whether the file was changed. Versioning file system: A versioning filesystem tracks all changes to a file. The NILFS versioning filesystem for Linux is an example.

Not all information stored on the computer is stored in files. Accurately recovering a complete system from scratch requires keeping track of this non file data too. System description: System specifications are needed to procure an exact replacement after a disaster. Boot sector: The boot sector can sometimes be recreated more easily than saving it. It usually isn't a normal file and the system won't boot without it. Partition layout: The layout of the original disk, as well as partition tables and filesystem settings, is needed to properly recreate the original system. File metadata: Each file's permissions, owner, group, ACLs, and any other metadata need to be backed up for a restore to properly recreate the original environment. System metadata: Different operating systems have different ways of storing configuration information. Microsoft Windows keeps a registry of system information that is more difficult to restore than a typical file.

It is frequently useful or required to manipulate the data being backed up to optimize the backup process. These manipulations can improve backup speed, restore speed, data security, media usage and/or reduced bandwidth requirements.

Out of date data can be automatically deleted, but for personal backup applications—as opposed to enterprise client server backup applications where automated data "grooming" can be customized—the deletion can at most be globally delayed or be disabled.

Various schemes can be employed to shrink the size of the source data to be stored so that it uses less storage space. Compression is frequently a built in feature of tape drive hardware.

Redundancy due to backing up similarly configured workstations can be reduced, thus storing just one copy. This technique can be applied at the file or raw block level. This potentially large reduction Encrypting the data on these media can mitigate this problem, however encryption is a CPU intensive process that can slow down backup speeds, and the security of the encrypted backups is only as effective as the security of the key management policy. However cramming the scheduled backup window via "multiplexed backup" is only used for tape destinations. Recovery time objective (RTO): The amount of time elapsed between disaster and restoration of business functions. Data security: In addition to preserving access to data for its owners, data must be restricted from unauthorized access. Backups must be performed in a manner that does not compromise the original owner's undertaking. This can be achieved with data encryption and proper media handling policies. Data retention period: Regulations and policy can lead to situations where backups are expected to be retained for a particular period, but not any further. Retaining backups after this period can lead to unwanted liability and sub optimal use of storage media. Backup process monitoring: Enterprise client server backup applications need a user interface that allows administrators to monitor the backup process, and proves compliance to regulatory bodies outside the organization; for example, an insurance company in the USA might be required under HIPAA to demonstrate that its client data meet records retention requirements. User initiated backups and restores: To avoid or recover from minor disasters, such as inadvertently deleting or overwriting the "good" versions of one or more files, the computer user—rather than an administrator—may initiate backups and restores (from not necessarily the most recent backup) of files or folders.