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{{Short description|Stored data in computer systems}}{{about|duplicate data in computer systems for data recovery|other uses}}{{Use dmy dates|date=October 2019}}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 “(wikt:back up|back up)”, whereas the noun and adjective form is “(wikt:backup|backup)”.WEB, back•up,www.ahdictionary.com/word/search.html?q=backup, The American Heritage Dictionary of the English Language, Houghton Mifflin Harcourt, 9 May 2018, 2018, Backups can be used to recover data after its loss from data deletion or corruption, or to recover data from an earlier time.{{Nbsp|2=hair}}BOOK 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.In contrast to everyday use of the term “archive”, the data stored in an “archive file” is not necessarily old or of historical interest.BOOK 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 limitationsNEWS, The New York Times

Storage

A backup strategy requires an information repository, “a secondary storage space for data“WEB, McMahon, Mary, What Is an Information Repository?,www.wisegeek.com/what-is-an-information-repository.htm, wiseGEEK, Conjecture Corporation, 8 May 2019, 1 April 2019, In the sense of an approach to data management, an information repository is 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, 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.

Backup methods

Unstructured

An unstructured repository may simply be a stack of tapes, DVD-Rs or external HDDs with minimal information about what was backed up and when. This method is the easiest to implement, but unlikely to achieve a high level of recoverability as it lacks automation.

Full only/System imaging

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, imagingWEB, Five key questions to ask about your backup solution,sysgen.ca/five-key-backup-questions/, sysgen.ca, 23 September 2015,sysgen.ca/five-key-backup-questions/," title="web.archive.org/web/20160304042343sysgen.ca/five-key-backup-questions/,">web.archive.org/web/20160304042343sysgen.ca/five-key-backup-questions/, 4 March 2016, live, dmy-all, Does your company have a low tolerance to longer “data access outages” and/or would you like to minimize the time your company may be without its data?, 23 March 2014, 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.

Incremental

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.WEB, Incremental Backup,www.tech-faq.com/incremental-backup.shtml, Tech-FAQ, Independent Media, 10 March 2006,www.tech-faq.com/incremental-backup.shtml," title="web.archive.org/web/20160621090117www.tech-faq.com/incremental-backup.shtml,">web.archive.org/web/20160621090117www.tech-faq.com/incremental-backup.shtml, 21 June 2016, 13 June 2005, Some backup systemsWEB, Pond, James,baligu.com/pondini/TM/Works.html, How Time Machine Works its Magic, Apple OSX and Time Machine Tips, baligu.com, 19 May 2019, 31 August 2013, File System Event Store, Hard Links, 21 June 2019,web.archive.org/web/20190621092705/https://www.baligu.com/pondini/TM/Works.html, dead, can create a {{visible anchor|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.

Near-CDP{{anchor|Continuous_data_protection}}

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.WEB Near-CDP (except for Apple Time Machine)WEB, Pond, James, How Time Machine Works its Magic,www.baligu.com/pondini/TM/Works.html, Apple OSX and Time Machine Tips, Baligu.com (as mirrored after James Pond died in 2013), 10 July 2019, 31 August 2013, The File System Event Store is a hidden log that OSX keeps on each HFS+ formatted disk/partition of changes made to the data on it. It doesn’t list every file that’s changed, but each directory (folder) that’s had anything changed inside it., intent-logs every change on the host system,BOOK,books.google.com/books?id=2OtqvySBTu4C&pg=PA287, Enterprise Systems Backup and Recovery: A Corporate Insurance Policy, de Guise, P., CRC Press, 285–287, 2009, 978-1-4200-7639-4, 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 machineWEB, Wu, Victor, EMC RecoverPoint for Virtual Machine Overview,wuchikin.wordpress.com/2017/03/04/emc-recoverpoint-for-virtual-machine-overview/, Victor Virtual, WuChiKin, 22 June 2019, 4 March 2017, The splitter splits out the Write IOs to the VMDK/RDM of a VM and sends a copy to the production VMDK and also to the RecoverPoint for VMs cluster., WEB, Zerto or Veeam?,resqdr.com/zerto-or-veeam/, RES-Q Services, 7 July 2019, March 2017, Zerto doesn’t use snapshot technology like Veeam. Instead, Zerto deploys small virtual machines on its physical hosts. These Zerto VMs capture the data as it is written to the host and then send a copy of that data to the replication site.....However, Veeam has the advantage of being able to more efficiently capture and store data for long-term retention needs. There is also a significant pricing difference, with Veeam being cheaper than Zerto., or equivalentWEB, Agent Related,docs.cloudendure.com/Content/FAQ/FAQ/Agent_Related.htm, CloudEndure.com, 3 July 2019, What does the CloudEndure Agent do?, 2019, The CloudEndure Agent performs an initial block-level read of the content of any volume attached to the server and replicates it to the Replication Server. The Agent then acts as an OS-level read filter to capture writes and synchronizes any block level modifications to the CloudEndure Replication Server, ensuring near-zero RPO., 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.

Reverse incremental

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.

Differential

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.

Storage media

File:DVD, USB flash drive and external hard drive.jpg|thumb|right|From left to right, a DVD disc in plastic cover, a USB flash drive and an external hard driveexternal hard driveRegardless 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

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.WEB, 9 December 2004, 26 May 2019 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.WEB, Digital Data Storage Outlook 2017,spectralogic.com/wp-content/uploads/white-paper-digital-data-storage-outlook-2017-v3.pdf, Spectra, Spectra Logic, 11 July 2018, 7(Solid-State), 10(Magnetic Disk), 14(Tape), 17(Optical), 2017, 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.WEB, Tom Coughlin, Keeping Data for a Long Time,www.forbes.com/sites/tomcoughlin/2014/06/29/keeping-data-for-a-long-time/, Forbes, 19 April 2018, 29 June 2014, para. Magnetic Tapes(popular formats, storage life), para. Hard Disk Drives(active archive), para. First consider flash memory in archiving(... may not have good media archive life),

Hard disk

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. However, hard disk backups are close-tolerance mechanical devices and may be more easily damaged than tapes, especially while being transported.WEB, Jacobi, John L., Hard-core data preservation: The best media and methods for archiving your data,www.pcworld.com/article/2984597/storage/hard-core-data-preservation-the-best-media-and-methods-for-archiving-your-data.html, PC World, 19 April 2018, 29 February 2016, sec. External Hard Drives(on the shelf, magnetic properties, mechanical stresses, vulnerable to shocks), Tape, Online storage, In the mid-2000s, several drive manufacturers began to produce portable drives employing ramp loading and accelerometer technology (sometimes termed a “shock sensor“),WEB, Ramp Load/Unload Technology in Hard Disk Drives,www.hgst.com/sites/default/files/resources/LoadUnload_white_paper_FINAL.pdf, HGST, Western Digital, 29 June 2018, 3(sec. Enhanced Shock Tolerance), November 2007, WEB, Toshiba Portable Hard Drive (Canvio® 3.0),www.toshibadata.com.sg/Product-Canvio-Portable-Hard-Drive.aspx, Toshiba Data Dynamics Singapore, Toshiba Data Dynamics Pte Ltd, 16 June 2018, 2018, sec. Overview(Internal shock sensor and ramp loading technology), 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.WEB, Iomega Drop Guard â„¢ Technology,www.doc-developpement-durable.org/file/Projets-informatiques/Drop%20Guard-disque-dur-tres-solide.pdf, Hard Drive Storage Solutions, Iomega Corp., 12 July 2018, 2(What is Drop Shock Technology?, What is Drop Guard Technology? (... features special internal cushioning .... 40% above the industry average)), 3(*NOTE), 20 September 2010, 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.WEB, John Burek 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.BOOK, Kissell, Joe, Take Control of Backing Up Your Mac, 2011, TidBITS Publishing Inc., Ithaca NY, 978-1-61542-394-1, 41(Deduplication),books.google.com/books?id=ANe3k_7bnAcC&q=retrospect+deduplication&pg=PT41, 17 September 2019, WEB,www.dcig.com/2009/07/symantec-shows-backup-exec-a-l.html, Symantec Shows Backup Exec a Little Dedupe Love; Lays out Source Side Deduplication Roadmap – DCIG, DCIG, 7 July 2009, 26 February 2016,www.dcig.com/2009/07/symantec-shows-backup-exec-a-l.html," title="web.archive.org/web/20160304212819www.dcig.com/2009/07/symantec-shows-backup-exec-a-l.html,">web.archive.org/web/20160304212819www.dcig.com/2009/07/symantec-shows-backup-exec-a-l.html, 4 March 2016, live, dmy-all, WEB, Veritas NetBackupâ„¢ Deduplication Guide,www.veritas.com/content/support/en_US/doc/ka6j00000000ADEAA2, Veritas, Veritas Technologies LLC, 26 July 2018, 2016,

Optical storage

(File:Water running on CD-RW - label side.jpg|thumb|Optical discs are not vulnerable to water, making them likely to survive a flood disaster.){{See also|Optical media preservation}}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.JOURNAL