Editing Backup (section)

===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 image]]s, this method is frequently used by computer technicians to record known good configurations. However, imaging<ref>{{Cite web |title=Five key questions to ask about your backup solution |url=http://sysgen.ca/five-key-backup-questions/ |website=sysgen.ca |access-date=23 September 2015 |archive-url=https://web.archive.org/web/20160304042343/http://sysgen.ca/five-key-backup-questions/ |archive-date=4 March 2016 |url-status=live |df=dmy-all|at=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?|date=23 March 2014 }}</ref> 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.<ref name="Tech-FAQIncrementalBackup">{{cite web |title=Incremental Backup |url=http://www.tech-faq.com/incremental-backup.shtml |website=Tech-FAQ |publisher=Independent Media |access-date=10 March 2006 |archive-url=https://web.archive.org/web/20160621090117/http://www.tech-faq.com/incremental-backup.shtml |archive-date=21 June 2016 |date=13 June 2005}}</ref>
Some backup systems<ref name="PondHowTimeMachineWorks">{{cite web | last1=Pond | first1=James | url=http://baligu.com/pondini/TM/Works.html | title=How Time Machine Works its Magic | website=Apple OSX and Time Machine Tips | publisher=baligu.com | access-date=19 May 2019 | date=31 August 2013 | at=File System Event Store, Hard Links | archive-date=21 June 2019 | archive-url=https://web.archive.org/web/20190621092705/https://www.baligu.com/pondini/TM/Works.html | url-status=dead }}</ref> 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.<ref name=InformationWeekWhyCDPGettingMorePractical>{{cite web
   |author=Behzad Behtash
   |url=https://www.informationweek.com/why-continuous-data-protections-getting-more-practical/d/d-id/1088883
   |title=Why Continuous Data Protection's Getting More Practical
   |work=Disaster recovery/business continuity
   |publisher=InformationWeek |date=6 May 2010 |access-date=12 November 2011 |quote=A true CDP approach should capture all data writes, thus continuously backing up data and eliminating backup windows.... CDP is the gold standard—the most comprehensive and advanced data protection. But "near CDP" technologies can deliver enough protection for many companies with less complexity and cost. For example, snapshots can provide a reasonable near-CDP-level of protection for file shares, letting users directly access data on the file share at regular intervals--say, every half-hour or 15 minutes. That's certainly a higher level of protection than tape-based or disk-based nightly backups and may be all you need.}}</ref> Near-CDP backup applications—often [[List of backup software#Proprietary|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.<ref name=InformationWeekWhyCDPGettingMorePractical /> Near-CDP backup applications use [[Journaling file system|journaling]] and are typically based on periodic "snapshots",<ref name="ComputerWeeklyCDPExplained">{{cite web |title=Continuous data protection (CDP) explained: True CDP vs near-CDP |url=https://www.computerweekly.com/Continuous-data-protection-CDP-explained-True-CDP-vs-near-CDP |website=ComputerWeekly.com |publisher=TechTarget |access-date=22 June 2019 |date=July 2010 |quote=... copies data from a source to a target. True CDP does this every time a change is made, while so-called near-CDP does this at pre-set time intervals. Near-CDP is effectively the same as snapshotting....True CDP systems record every write and copy them to the target where all changes are stored in a log. [new paragraph] By contrast, near-CDP/snapshot systems copy files in a straightforward manner but require applications to be quiesced and made ready for backup, either via the application's backup mode or using, for example, Microsoft's Volume Shadow Copy Services (VSS).}}</ref> [[file system permissions|read-only]] copies of the data frozen at a particular [[point in time]].

Near-CDP (except for [[Apple Time Machine]])<ref name="PondiniHowTMWorksItsMagic">{{cite web |last1=Pond |first1=James |title=How Time Machine Works its Magic |url=https://www.baligu.com/pondini/TM/Works.html |website=Apple OSX and Time Machine Tips |publisher=Baligu.com (as mirrored after James Pond died in 2013) |access-date=10 July 2019 |date=31 August 2013 |quote=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.}}</ref> [[Intent log|intent-logs]] every change on the host system,<ref name="deGuiseEnterprise09#A.3.3">{{cite book |url=https://books.google.com/books?id=2OtqvySBTu4C&pg=PA287|title=Enterprise Systems Backup and Recovery: A Corporate Insurance Policy |author=de Guise, P. |publisher=CRC Press |pages=285–287 |year=2009 |isbn=978-1-4200-7639-4}}</ref> 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<ref name="VictorWuEMCRecoverPointVM">{{cite web |last1=Wu |first1=Victor |title=EMC RecoverPoint for Virtual Machine Overview |url=https://wuchikin.wordpress.com/2017/03/04/emc-recoverpoint-for-virtual-machine-overview/ |website=Victor Virtual |publisher=WuChiKin |access-date=22 June 2019 |date=4 March 2017 |quote=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.}}</ref><ref name="RES-QServicesZertoOrVeeam">{{cite web |title=Zerto or Veeam? |url=https://resqdr.com/zerto-or-veeam/ |website=RES-Q Services |access-date=7 July 2019 |date=March 2017 |quote=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.}}</ref> or equivalent<ref name="CloudEndureAgentRelated">{{cite web |title=Agent Related |url=https://docs.cloudendure.com/Content/FAQ/FAQ/Agent_Related.htm |website=CloudEndure.com |access-date=3 July 2019 |at=What does the CloudEndure Agent do? |date=2019 |quote=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.}}</ref> 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 file|".bak" extension]] to the [[file name]].

====Reverse incremental====
A [[Incremental backup#Reverse incremental|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.