Backup

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.[note 1][4] 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[5] and human factors involved in any backup scheme.

Backup

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A backup strategy requires an information repository, "a secondary storage space for data"[6] 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,[4] which is a system of backing up data to computer media that limits the number of backups of different dates retained separately, by appropriate re-use of the data storage media by overwriting of backups no longer needed. The scheme determines how and when each piece of removable storage is used for a backup operation and how long it is retained once it has backup data stored on it. The 3-2-1 rule can aid in the backup process. It states that there should be at least 3 copies of the data, stored on 2 different types of storage media, and one copy should be kept offsite, in a remote location (this can include cloud storage). 2 or more different media should be used to eliminate data loss due to similar reasons (for example, optical discs may tolerate being underwater while LTO tapes may not, and SSDs cannot fail due to head crashes or damaged spindle motors since they do not have any moving parts, unlike hard drives). An offsite copy protects against fire, theft of physical media (such as tapes or discs) and natural disasters like floods and earthquakes. Physically protected hard drives are an alternative to an offsite copy, but they have limitations like only being able to resist fire for a limited period of time, so an offsite copy still remains as the ideal choice.

A repository using this backup method contains complete source data copies taken at one or more specific points in time. Copying system images, this method is frequently used by computer technicians to record known good configurations. However, imaging[7] 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.[8]Some backup systems[9] can create a .mw-parser-output .vanchor>:target.vanchor-textbackground-color:#b1d2ffsynthetic 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.

Near-CDP (except for Apple Time Machine)[12] intent-logs every change on the host system,[13] 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[14][15] or equivalent[16] and is therefore generally used in enterprise client-server backups.

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.

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.[17] 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.

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.[18] However, hard disk backups are close-tolerance mechanical devices and may be more easily damaged than tapes, especially while being transported.[20] In the mid-2000s, several drive manufacturers began to produce portable drives employing ramp loading and accelerometer technology (sometimes termed a "shock sensor"),[21][22] 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.[23] 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.[23][24][25] Over a period of years the stability of hard disk backups is shorter than that of tape backups.[19][26][20]

External hard disks can be connected via local interfaces like SCSI, USB, FireWire, or eSATA, or via longer-distance technologies like Ethernet, iSCSI, or Fibre Channel. Some disk-based backup systems, via Virtual Tape Libraries or otherwise, support data deduplication, which can reduce the amount of disk storage capacity consumed by daily and weekly backup data.[27][28][29]

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.[37][24] Flash memory backups are stable for fewer years than hard disk backups.[19]

Remote backup services or cloud backups involve service providers storing data offsite. This has been used to protect against events such as fires, floods, or earthquakes which could destroy locally stored backups.[38] Cloud-based backup (through services like or similar to Google Drive, and Microsoft OneDrive) provides a layer of data protection.[20] However, the users must trust the provider to maintain the privacy and integrity of their data, with confidentiality enhanced by the use of encryption. Because speed and availability are limited by a user's online connection,[20] users with large amounts of data may need to use cloud seeding and large-scale recovery.

Various methods can be used to manage backup media, striking a balance between accessibility, security and cost. These media management methods are not mutually exclusive and are frequently combined to meet the user's needs. Using on-line disks for staging data before it is sent to a near-line tape library is a common example.[39][40]

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. 2990b30de8