In information technology, backup refers to making copies of data so that these additional copies may be used to restore the original after a data loss event. These additional copies are typically called "backups." Backups are useful primarily for two purposes. The first is to restore a state following a disaster (called disaster recovery). The second is to restore small numbers of files after they have been accidentally deleted or corrupted. Data loss is also very common. 66% of internet users have suffered from serious data loss.
Since a backup system contains at least one copy of all data worth saving, the data storage requirements are considerable. Organizing this storage space and managing the backup process is a complicated undertaking. A data repository model can be used to provide structure to the storage. In the modern era of computing there are many different types of data storage devices that are useful for making backups. There are also many different ways in which these devices can be arranged to provide geographic redundancy, data security, and portability.
Before data is sent to its storage location, it is selected, extracted, and manipulated. Many different techniques have been developed to optimize the backup procedure. These include optimizations for dealing with open files and live data sources as well as compression, encryption, and de-duplication, among others. Many organizations and individuals try to have confidence that the process is working as expected and work to define measurements and validation techniques. It is also important to recognize the limitations and human factors involved in any backup scheme.
- 1 Storage, the base of a backup system
- 2 Selection, extraction and manipulation of data
- 3 Managing the backup process
- 4 Lore
- 5 See also
- 6 References
Storage, the base of a backup system
Data repository models
Any backup strategy starts with a concept of a data repository. The backup data needs to be stored somehow and probably should be organized to a degree. It can be as simple as a sheet of paper with a list of all backup tapes and the dates they were written or a more sophisticated setup with a computerized index, catalog, or relational database. Different repository models have different advantages. This is closely related to choosing a backup rotation scheme.
- An unstructured repository may simply be a stack of floppy disks or CD-R/DVD-R media with minimal information about what was backed up and when. This is the easiest to implement, but probably the least likely to achieve a high level of recoverability.
- Full + Incrementals
- A Full + Incremental repository aims to make storing several copies of the source data more feasible. At first, a full backup (of all files) is taken. After that, any number of incremental backups can be taken. There are many different types of incremental backups, but they all attempt to only backup a small amount of data relative to the full backup. Restoring a whole system to a certain point in time would require locating the full backup taken previous to that time and the incremental backups that cover the period of time between the full backup and the particular point in time to which the system is supposed to be restored. The scope of an incremental backup is typically defined as a range of time relative to other full or incremental backups. Different implementations of backup systems frequently use specialized or conflicting definitions of these terms.
- Continuous data protection
- This model takes it a step further and instead of scheduling periodic backups, the system immediately logs every change on the host system. This is generally done by saving byte or block-level differences rather than file-level differences. It differs from simple disk mirroring in that it enables a roll-back of the log and thus restoration of old image of data.
Regardless of the repository model that is used, the data has to be stored on some data storage medium somewhere.
- Magnetic tape
- Magnetic tape has long been the most commonly used medium for bulk data storage, backup, archiving, and interchange. Tape has typically had an order of magnitude better capacity/price ratio when compared to hard disk, but recently the ratios for tape and hard disk have become a lot closer. There are myriad formats, many of which are proprietary or specific to certain markets like mainframes or a particular brand of personal computer. Tape is a sequential access medium, so even though access times may be poor, the rate of continuously writing or reading data can actually be very fast. Some new tape drives are even faster than modern hard disks. A principal advantage of tape is that it has been used for this purpose for decades (much longer than any alternative) and its characteristics are well understood.
- Hard disk
- The capacity/price ratio of hard disk has been rapidly improving for many years. This is making it more competitive with magnetic tape as a bulk storage medium. The main advantages of hard disk storage are low access times, availability, capacity and ease of use. External 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, such as Virtual Tape Libraries, support data deduplication which can dramatically reduce the amount of disk storage capacity consumed by daily and weekly backup data. The main disadvantages of hard disk backups are that they are easily damaged, especially while being transported (e.g., for off-site backups), and that their stability over periods of years is a relative unknown.
- Optical disc
- A recordable CD can be used as a backup device. One advantage of CDs is that they can in theory be restored on any machine with a CD-ROM drive. (In practice, writable CD-ROMs are not always universally readable.) In addition, recordable CD's are relatively cheap. Another common format is recordable DVD. Many optical disk formats are WORM type, which makes them useful for archival purposes since the data can't be changed. Other rewritable formats can also be utilized such as CD-RW or DVD-RAM. The newer HD-DVDs and Blu-ray Discs dramatically increase the amount of data possible on a single optical storage disk, though, as yet, the hardware may be cost prohibitive for many people. Additionally the physical lifetime of the optical disk has become a concern as it is possible for some optical disks to degrade and lose data within a couple of years.
- Floppy disk
- During the 1980s and early 1990s, many personal/home computer users associated backup mostly with copying floppy disks. The low data capacity of a floppy disk makes it an unpopular and obsolete choice today.
- Solid state storage
- Also known as flash memory, thumb drives, USB flash drives, CompactFlash, SmartMedia, Memory Stick, Secure Digital cards, etc., these devices are relatively costly for their low capacity, but offer excellent portability and ease-of-use.
- Remote backup service
- As broadband internet access becomes more widespread, remote backup services are gaining in popularity. Backing up via the internet to a remote location can protect against some worst-case scenarios such as fires, floods, or earthquakes which would destroy any backups in the immediate vicinity along with everything else. There are, however, a number of drawbacks to remote backup services. First, internet connections (particularly domestic broadband connections) are generally substantially slower than the speed of local data storage devices, which can be a problem for people who generate or modify large amounts of data. Secondly, users need to trust a third party service provider with both privacy and integrity of backed up data. The risk associated with putting control of personal or sensitive data in the hands of a third party can be managed by encrypting sensitive data so that its contents cannot be viewed without access to the secret key. Note that ultimately the backup service must itself be using one of the above methods, so this could be seen as a more complex way of doing traditional backups.
Managing the data repository
Regardless of the data repository model or data storage media used for backups, a balance needs to be struck between accessibility, security and cost. These media management methods are not mutually exclusive and are frequently combined to meet the needs of the situation. Using on-line disks for staging data before it is sent to a near-line tape library is a common example.
- On-line backup storage is typically the most accessible type of data storage, which can begin restore in milliseconds time. A good example would be an internal hard disk or a disk array (maybe connected to SAN). This type of storage is very convenient and speedy, but is relatively expensive. On-line storage is quite vulnerable to being deleted or overwritten, either by accident, by intentional malevolent action, or in the wake of a data-deleting virus payload.
- Near-line storage is typically less accessible and less expensive than on-line storage, but still useful for backup data storage. A good example would be a tape library with restore times ranging from seconds to a few minutes. A mechanical device is usually involved in moving 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.
- Off-line storage requires some direct human action in order to make access to the storage media physically possible. This action is typically inserting a tape into a tape drive or plugging in a cable that allows a device to be accessed. Because the data is not accessible via any computer except during limited periods in which it is written or read back, it is largely immune to a whole class of on-line backup failure modes. Access time will vary depending on whether the media is on-site or off-site.
- Off-site Off-line vault
- To protect against a disaster or other site-specific problem, many people choose to send backup media to an off-site vault. The vault can be as simple as the System Administrator’s home office or as sophisticated as a disaster hardened, temperature controlled, high security bunker that has facilities for backup media storage. Note carefully that 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, and should not be confused with an off-line backup.
- Backup site, Disaster Recovery Center or DR Center
- In the event of a disaster, the data on backup media will not be sufficient to recover. Computer systems onto which the data can be restored and properly configured networks are necessary too. Some organizations have their own data recovery centers that are equipped for this scenario. Other organizations contract this out to a third-party recovery center. Note that because DR site is itself a huge investment, backup is very rarely considered preferred method of moving data to DR site. More typical way would be remote disk mirroring, which keeps the DR data as up-to-date as possible.
Selection, extraction and manipulation of data
Selection and extraction of file data
Deciding what to back up at any given time is a harder process than it seems. By backing up too much redundant data, the data 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
- Instead of copying whole files, one can limit the backup to only the blocks or bytes within a file that have changed in a given period of time. This technique can use substantially less storage space on the backup medium, but requires a high level of sophistication to reconstruct files in a restore situation. Some implementations require integration with the source filesystem.
- Filesystem dump
- Instead of copying files within a filesystem, a copy of the whole filesystem itself 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 dump. This type of backup has the possibility of running faster than a backup that simply copies files. A feature of some dump software is the ability to restore specific files from the dump image.
- 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 keeps track of all changes to a file and makes those changes accessible to the user. Generally this gives access to any previous version, all the way back to the file's creation time. An example of this is the Wayback versioning filesystem for Linux.
Selection and extraction of live data
If a computer system is in use while it is being backed up, the possibility of files being open for reading or writing is real. If a file is open, the contents on disk may not correctly represent what the owner of the file intends. This is especially true for database files of all kinds. 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 any single point in time. This is because the data being backed up changed in the period of time between when the backup started and when it finished. For databases in particular, fuzzy backups are worthless.
- Snapshot backup
- A snapshot is an instantaneous function of some storage systems that presents a copy of the filesystem as if it was frozen in a specific point in time, often by a copy-on-write mechanism. An effective way to back up live data is to temporarily quiesce it (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.  While a snapshot is very handy for viewing a filesystem as it was at a different point in time, it is hardly an effective backup mechanism by itself.
- Open file backup
- Many backup software packages feature the ability to handle open files in backup operations. Some simply check for openness and try again later. File locking is useful for regulating access to open files.
- When attempting to understand the logistics of backing up open files, one must consider that the backup process could take several minutes to back up a large file such as a database. In order to back up a file that is in use, it is vital that the entire backup represent a single-moment snapshot of the file, rather than a simple copy of a read-through. This represents a challenge when backing up a file that is constantly changing. Either the database file must be locked to prevent changes, or a method must be implemented to ensure that the original snapshot is preserved long enough to be copied, all while changes are being preserved. Backing up a file while it is being changed, in a manner that causes the first part of the backup to represent data before changes occur to be combined with later parts of the backup after the change results in a corrupted file that is unusable, as most large files contain internal references between their various parts that must remain consistent throughout the file.
- Cold database backup
- During a cold backup, the database is closed or locked and not available to users. The datafiles do not change during the backup process so the database is in a consistent state when it is returned to normal operation. 
- Hot database backup
- Some database management systems offer a means to generate a backup image of the database while it is online and usable ("hot"). This usually includes an inconsistent image of the data files plus a log of changes made while the procedure is running. Upon a restore, the changes in the log files are reapplied to bring the database in sync. 
Selection and extraction of metadata
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. Still, 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. Windows keeps a registry of system information that is more difficult to restore than a typical file.
Manipulation of data and dataset optimisation
It is frequently useful or required to manipulate the data being backed up to optimize the backup process. These manipulations provide many benefits including improved backup speed, restore speed, data security, media usage and reduced bandwidth requirements.
- 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.
- When multiple similar systems are backed up to the same destination storage device, there exists the potential for much redundancy within the backed up data. For example, if 20 Windows workstations were backed up to the same data repository, they might share a common set of system files. The data repository only needs to store one copy of those files to be able to restore any one of those workstations. This technique can be applied at the file level or even on raw blocks of data, potentially resulting in a massive reduction in required storage space. Deduplication can occur on a server before any data moves to backup media, sometimes referred to as source/client side deduplication. This approach also reduces bandwidth required to send backup data to its target media. The process can also occur at the target storage device, sometimes referred to as inline or back-end deduplication.
- Sometimes backup jobs are duplicated to a second set of storage media. This can be done to rearrange the backup images to optimize restore speed, to have a second copy at a different location or on a different storage medium.
- High capacity removable storage media such as backup tapes present a data security risk if they are lost or stolen.  Encrypting the data on these media can mitigate this problem, but presents new problems. First, encryption is a CPU intensive process that can slow down backup speeds. Second, once data has been encrypted, it can not be effectively compressed and the data compression function of many tape drives is ineffective. For this reason and since redundant data makes cryptanalytic attacks easier, many encryption implementations compress the data before encrypting it. Third, the security of the encrypted backups is only as effective as the security of the key management policy.
- When there are many more computers to be backed up than there are destination storage devices, the ability to use a single storage device with several simultaneous backups can be useful.
- The process of rearranging the backup sets in a data repository is known as refactoring. For example, if a backup system uses a single tape each day to store the incremental backups for all the protected computers, restoring one of the computers could potentially require many tapes. Refactoring could be used to consolidate all the backups for a single computer onto a single tape. This is especially useful for backup systems that do incrementals forever style backups.
- Sometimes backup jobs are copied to a staging disk before being copied to tape. This process is sometimes referred to as D2D2T, an acronym for Disk to Disk to Tape. This can be useful if there is a problem matching the speed of the final destination device with the source device as is frequently faced in network-based backup systems. It can also serve as a centralized location for applying other data manipulation techniques.
Managing the backup process
It is important to understand that backup is a process. As long as new data is being created and changes are being made, backups will need to be updated. Individuals and organizations with anything from one computer to thousands (or even millions) of computer systems all have requirements for protecting data. While the scale is different, the objectives and limitations are essentially the same. Likewise, those who perform backups need to know to what extent they were successful, regardless of scale.
- Recovery Point Objective (RPO)
- The point in time that the restarted infrastructure will reflect. Essentially, this is the roll-back that will be experienced as a result of the recovery. The most desirable RPO would be the point just prior to the data loss event. Making a more recent recovery point achievable requires increasing the frequency of synchronization between the source data and the backup repository.
- 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.
An effective backup scheme will take into consideration the limitations of the situation.
- Backup window
- The period of time when backups are permitted to run on a system is called the backup window. This is typically the time when the system sees the least usage and the backup process will have the least amount of interference with normal operations. The backup window is usually planned with users' convenience in mind. If a backup extends past the defined backup window, a decision is made whether it is more beneficial to abort the backup or to lengthen the backup window.
- Performance impact
- All backup schemes have some performance impact on the system being backed up. For example, for the period of time that a computer system is being backed up, the hard drive is busy reading files for the purposes of the backup, and its full bandwidth is no longer available for other tasks. Such impacts should be analyzed.
- Costs of hardware, software, labor
- All types of storage media have a finite capacity with a real cost. Matching the correct amount of storage capacity (over time) with the backup needs is an important part of the design of a backup scheme. Any backup scheme has some labor requirement, but complicated schemes have considerably higher labor requirements. The cost of commercial backup software can also be considerable.
- Network Bandwidth
- Distributed backup systems can be affected by limited network bandwidth.
Meeting the defined objectives in the face of the above limitations can be a difficult task. The tools and concepts below can make that task more achievable.
- Using a Job scheduler can greatly improve the reliability and consistency of backups by removing part of the human element. Many backup software packages include this functionality.
- Over the course of regular operations, the user accounts and/or system agents that perform the backups need to be authenticated at some level. The power to copy all data off of or onto a system requires unrestricted access. Using an authentication mechanism is a good way to prevent the backup scheme from being used for unauthorized activity.
- Chain of trust
- Removable storage media are physical items and must only be handled by trusted individuals. Establishing a chain of trusted individuals (and vendors) is critical to defining the security of the data.
Measuring the process
To ensure that the backup scheme is working as expected, the process needs to include monitoring key factors and maintaining historical data.
- Backup validation
- (also known as "Backup Success Validation") The process by which owners of data can get information regarding how their data was backed up. This same process is also used to prove compliance to regulatory bodies outside of the organization, for example, an insurance company might be required under HIPAA to show "proof" that their patient data are meeting records retention requirements. Disaster, data complexity, data value and increasing dependence upon ever-growing volumes of data all contribute to the anxiety around and dependence upon successful backups to ensure business continuity. For that reason, many organizations rely on third-party or "independent" solutions to test, validate, and optimize their backup operations (backup reporting).
- In larger configurations, reports are useful for monitoring media usage, device status, errors, vault coordination and other information about the backup process.
- In addition to the history of computer generated reports, activity and change logs are useful for monitoring backup system events.
- Many backup programs make use of checksums or hashes to validate that the data was accurately copied. These offer several advantages. First, they allow data integrity to be verified without reference to the original file: if the file as stored on the backup medium has the same checksum as the saved value, then it is very probably correct. Second, some backup programs can use checksums to avoid making redundant copies of files, to improve backup speed. This is particularly useful for the de-duplication process.
- Monitored Backup
- Backup processes are monitored by a third party monitoring center. This center alerts users to any errors that occur during automated backups. Monitored backup requires software capable of pinging the monitoring center's servers in the case of errors.
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Due to a considerable overlap in technology, backups and backup systems are frequently confused with archives and fault-tolerant systems. Backups differ from archives in the sense that archives are the primary copy of data, usually put away for future use, while backups are a secondary copy of data, kept on hand to replace the original item. Backup systems differ from fault-tolerant systems in the sense that backup systems assume that a fault will cause a data loss event and fault-tolerant systems assume a fault will not.
- The more important the data that is stored on the computer the greater the need is for backing up this data.
- A backup is only as useful as its associated restore strategy.
- Storing the copy near the original is unwise, since many disasters such as fire, flood and electrical surges are likely to cause damage to the backup at the same time.
- Automated backup and scheduling should be considered, as manual backups can be affected by human error.
- Backups will fail for a wide variety of reasons. A verification or monitoring strategy is an important part of a successful backup plan.
- It is good to store backed up archives in open/standard formats. This helps with recovery in the future when the software used to make the backup is obsolete. It also allows different software to be used.
- In 1996, during a fire at the headquarters of Credit Lyonnais, a major bank in Paris, system administrators ran into the burning building to rescue backup tapes because they didn't have offsite copies. Crucial bank archives and computer data were lost.  
- Privacy Rights Clearinghouse has documented  16 instances of stolen or lost backup tapes (among major organizations) in 2005 & 2006. Affected organizations included Bank of America, Ameritrade, Citigroup, and Time Warner.
- On 3 January 2008, an email server crashed at TeliaSonera, a major Nordic telecom company and internet service provider. It was subsequently discovered that the last serviceable backup set was from 15 December 2007. Three hundred thousand customer email accounts were affected. 
- Glossary of backup terms
- Backup software
- List of backup software
- Backup rotation scheme
- Incremental backup
- Computer data storage
- Data proliferation
- File synchronization
- Information repository
- Disaster recovery and business continuity auditing
- Digital preservation
- Reversible computing
- Data Recovery
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- "...destruction of crucial bank archives..."
- A Chronology of Data Breaches Privacy Rights Clearinghouse, San Diego
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- Telia Sonera to compensate clients over email crash. 2008-01-07. http://www.monstersandcritics.com/news/business/news/article_1385007.php/Telia_Sonera_to_compensate_clients_over_email_crash. Retrieved 2009-02-19.