Editing Backup (section)

==Selection and extraction of data==
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 [[Computer file|files]]. These files are organized into [[filesystem]]s. 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.<ref name="LeesWhatTo17">{{cite web |url=https://irontree.co.za/what-to-backup-a-critical-look-at-your-data-1935.html |title=What to backup – a critical look at your data |author=Lee
   |work=Irontree Blog |publisher=Irontree Internet Services CC
   |date=25 January 2017 |access-date=8 May 2018}}</ref>

===Files===
*[[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."<ref name="KissellTakeControlMacOSX" />

===Filesystems===
*Filesystem dump: A copy of the whole filesystem in block-level can be made. This is also known as a "raw partition backup" and is related to [[disk imaging]]. The process usually involves unmounting the filesystem and running a program like [[dd (Unix)]].<ref name="PrestonBackup07">{{cite book |url=https://books.google.com/books?id=6-w4fXbBInoC&pg=PA111 |title=Backup & Recovery: Inexpensive Backup Solutions for Open Systems |author=Preston, W.C. |publisher=O'Reilly Media, Inc |pages=111–114 |year=2007 |isbn=978-0-596-55504-7 |access-date=8 May 2018}}</ref> 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 [[filesystem]]s, 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.<ref name="PrestonUnix99">{{cite book |url=https://archive.org/details/unixbackuprecove00wcur |url-access=registration |title=Unix Backup & Recovery |author=Preston, W.C. |publisher=O'Reilly Media, Inc |pages=[https://archive.org/details/unixbackuprecove00wcur/page/73 73]–91 |year=1999 |isbn=978-1-56592-642-4 |access-date=8 May 2018}}</ref>
*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.<ref name="NILFSHome">{{cite web |title=NILFS Home |url=https://nilfs.sourceforge.io/en/ |website=NILFS Continuous Snapshotting System |publisher=NILFS Community |access-date=22 August 2019 |date=2019}}</ref>

===Live data===
Files that are actively being updated present a challenge to back up. One way to back up live data is to temporarily [[quiesce]] them (e.g., close all files), take a "snapshot", and then resume live operations. At this point the snapshot can be backed up through normal methods.<ref name="CougiasTheBackup03Chapter11">{{cite book |chapter-url=https://books.google.com/books?id=eLviiTag5A0C&pg=PA360|title=The Backup Book: Disaster Recovery from Desktop to Data Center |chapter=Chapter 11: Open file backup for databases |author=Cougias, D.J. |author2=Heiberger, E.L. |author3=Koop, K. |publisher=Network Frontiers |pages=356–360 |year=2003 |isbn=0-9729039-0-9}}</ref> A [[Snapshot (computer storage)|snapshot]] is an instantaneous function of some filesystems that presents a copy of the filesystem as if it were frozen at a specific point in time, often by a [[copy-on-write]] mechanism. Snapshotting a file while it is being changed results in a corrupted file that is unusable. This is also the case across interrelated files, as may be found in a conventional database or in applications such as [[Microsoft Exchange Server]].<ref name="ComputerWeeklyCDPExplained" /> The term [[fuzzy backup]] can be used to describe a backup of live data that looks like it ran correctly, but does not represent the state of the data at a single point in time.<ref name="LiotineMission03">{{cite book |url=https://books.google.com/books?id=LecC2BhPPxMC&pg=PA244 |title=Mission-critical Network Planning |author=Liotine, M. |publisher=Artech House |page=244 |year=2003 |isbn=978-1-58053-559-5 |access-date=8 May 2018}}</ref>

Backup options for data files that cannot be or are not quiesced include:<ref name="deGuiseEnterprise09#3.4.7">{{cite book |url=https://books.google.com/books?id=2OtqvySBTu4C&pg=PA50 |title=Enterprise Systems Backup and Recovery: A Corporate Insurance Policy |author=de Guise, P. |publisher=CRC Press |pages=50–54 |year=2009 |isbn=978-1-4200-7639-4}}</ref>
*Open file backup: Many backup software applications undertake to back up open files in an internally consistent state.<ref name="HandyBackupOpenFileWindows">{{cite web |title=Open File Backup Software for Windows |url=https://www.handybackup.net/open-file-backup.shtml
   |website=Handy Backup |publisher=Novosoft LLC
   |access-date=29 November 2018 |date=8 November 2018}}</ref> Some applications simply check whether open files are in use and try again later.<ref name="CougiasTheBackup03Chapter11" /> Other applications exclude open files that are updated very frequently.<ref name="ArqTroubleshootingBackingUpOpen/lockedFiles">{{cite web |last1=Reitshamer |first1=Stefan |title=Troubleshooting backing up open/locked files on Windows |url=https://www.arqbackup.com/blog/troubleshooting-backing-up-openlocked-files-on-windows/ |website=Arq Blog
   |publisher=Haystack Software
   |access-date=29 November 2018 |date=5 July 2017 |at=Stefan Reitshamer is the principal developer of Arq}}</ref> Some [[High availability|low-availability]] interactive applications can be backed up via natural/induced pausing.
*Interrelated database files backup: Some interrelated database file systems offer a means to generate a "hot backup"<ref name="UWiscOracleBackups">{{cite web |last1=Boss |first1=Nina |title=Oracle Tips Session #3: Oracle Backups |url=http://www.wisc.edu/drmt/oratips/sess003.html#Hotbackup |archive-url=https://web.archive.org/web/20070302110933/http://www.wisc.edu/drmt/oratips/sess003.html#Hotbackup |url-status=dead |website=www.wisc.edu |publisher=University of Wisconsin |access-date=1 December 2018 |archive-date=2 March 2007 |date=10 December 1997}}</ref> of the database while it is online and usable. This may include a snapshot of the data files plus a snapshotted log of changes made while the backup is running. Upon a restore, the changes in the log files are applied to bring the copy of the database up to the point in time at which the initial backup ended.<ref name="ArcserveOracleWhatIsARCHIVE-LOG">{{cite web |title=What is ARCHIVE-LOG and NO-ARCHIVE-LOG mode in Oracle and the advantages & disadvantages of these modes? |url=https://support.arcserve.com/s/article/202080249?language=en_US |website=Arcserve Backup |publisher=Arcserve |access-date=29 November 2018 |date=27 September 2018}}</ref> Other low-availability interactive applications can be backed up via coordinated snapshots. However, [[High availability|genuinely-high-availability]] interactive applications can be only be backed up via Continuous Data Protection.

===Metadata===
Not all information stored on the computer is stored in files. Accurately recovering a complete system from scratch requires keeping track of this [[metadata|non-file data]] too.<ref name="Gresovnik1">{{cite web |url=http://www2.arnes.si/~ljc3m2/igor/blogs/technical/bootable_media_creation.html |title=Preparation of Bootable Media and Images |last=Grešovnik |first=Igor |date=April 2016 |archive-url=https://web.archive.org/web/20160425113119/http://www2.arnes.si/~ljc3m2/igor/blogs/technical/bootable_media_creation.html |archive-date=25 April 2016 |access-date=21 April 2016}}</ref>
*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.
*[[Disk partitioning|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 [[Windows Registry|registry]] of system information that is more difficult to restore than a typical file.