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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 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 an incremental backup (of only the files that have changed since the previous full or incremental backup) can be taken. Restoring whole systems to a certain point in time would require locating the full backup taken previous to that time and all the incremental backups taken between that full backup and the particular point in time to which the system is supposed to be restored. This model offers a high level of security that something can be restored and can be used with removable media such as tapes and optical disks. The downside is dealing with a long series of incrementals and the high storage requirements.
Full + Differential 
A full + differential backup differs from a full + incremental in that after the full backup is taken, each partial backup captures all files created or changed since the full backup, even though some may have been included in a previous partial backup. Its advantage is that a restore involves recovering only the last full backup and then overlaying it with the last differential backup.
Mirror + Reverse Incrementals
A Mirror + Reverse Incrementals repository is similar to a Full + Incrementals repository. The difference is instead of an aging full backup followed by a series of incrementals, this model offers a mirror that reflects the system state as of the last backup and a history of reverse incrementals. One benefit of this is it only requires an initial full backup. Each incremental backup is immediately applied to the mirror and the files they replace are moved to a reverse incremental. This model is not suited to use removable media since every backup must be done in comparison to the mirror.
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 restore of old image of data.

Storage media

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.
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 or FireWire, or via longer distance technologies like Ethernet, iSCSI, or Fibre Channel. Some disk-based backup systems, such as Virtual Tape Libraries, support data de-duplication which can dramatically reduce the amount of disk storage capacity consumed by daily and weekly backup data.
Optical disc 
A recordable CD can be used as a backup device. One advantage of CDs is that they can be restored on any machine with a CD-ROM drive. As well, 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 CDRW or DVD-RAM. The newer HD-DVD's and BluRay Disks 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.
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 worse case scenarios, such as fire, flood or earthquake, destroying any backups along with everything else. A drawback to a remote backup service is that an internet connection is usually substantially slower than the speed of local data storage devices, so this can be a problem for people with large amounts of data. It also has the risk associated with putting control of personal or sensitive data in the hands of a third party.

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.

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 vulnerable to being deleted or overwritten, either by accident, 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.
Off-line storage is similar to near-line, except it requires human interaction to make storage media available. This can be as simple as storing backup tapes in a file cabinet. Media access time is more than an hour.
Off-site 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 Administratorís home office or as sophisticated as a disaster hardened, temperature controlled, high security bunker that has facilities for backup media storage.
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

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. If we don't back up enough data, critical information can get lost. The key concept is to only back up files that have changed.

Copying files 
Copy the files to be backed up to another location using the OS specific copy utility.
Filesystem dump 
Copy the filesystem that holds the files in question to another location. This usually involves unmounting the filesystem and running a program like dump. This is also known as a raw partition backup. 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.
Block Level Incremental 
A more sophisticated method of backing up changes to files is to only back up the blocks within the file that changed. This requires a higher level of integration between the filesystem and the backup software.
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.

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.

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 back up open files. Some simply check for openness and try again later. File locking is useful for regulating access to open files.
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 copy so the database is in sync upon restore.
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.
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.
Partition layout 
The layout of the original disk, as well as partition tables and filesystem settings, is needed to properly recreate the original system.
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.

Manipulation of data

It is frequently useful to manipulate the data being backed up to optimize the backup process. These manipulations can improve backup speed, restore speed, data security, and media usage.

Various schemes can be employed to shrink the size of the source data to be stored so that 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 (although since redundant data makes cryptanalytic attacks easier many encryption routines compress the data as an integral part of the encryption process). Third, the security of the encrypted backups is only as effective as the security of the key management policy.
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.


System impacts 
An effective backup scheme will take into consideration the limitations of the situation. All backup schemes have some impact on the system being backed up. If this impact is significant, the backup needs to be time-limited to a convenient backup window or alternate means of protecting data need to be employed. These alternate means tend to be more expensive.
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 impacted 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.



  • 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.

Glossary of backup terms

Backup policy 
An organisation's procedures and rules for ensuring that adequate amounts and types of backups are made, including suitably frequent testing of the process for restoring the original production system from the backup copies.
Backup rotation scheme 
A method for effectively backing up data where multiple media are systematically moved from storage to usage in the backup process and back to storage. There are several different schemes. Each takes a different approach to balance the need for a long retention period with frequently backing up changes. Some schemes are more complicated than others.
Backup site 
A place where business can continue after a data loss event. Such a site may have ready access to the backups or possibly even a continuously updated mirror.
Backup software 
Computer software applications that are used for performing the backing up of data, i.e., the systematic generation of backup copies.
Backup window 
The period of time that a system is available to perform a backup procedure. Backup procedures can have detrimental effects to system and network performance, sometimes requiring the primary use of the system to be suspended. These effects can be mitigated by arranging a backup window with the users or owners of the system(s).
Copy backup 
Term for full backup used by Windows Server 2003.
Cumulative incremental backup 
Term for a differential backup used by NetBackup.
Daily backup 
Term for incremental backup used by Windows Server 2003.
Data salvage 
The process of recovering data from storage devices when the normal operational methods are impossible. This process is typically performed by specialists in controlled environments with special tools. For example, a crashed hard disk may still have data on it even though it doesn't work properly. A data salvage specialist might be able to recover much of the original data by opening it up in a clean room and tinkering with the internal parts.
Differential backup 
A cumulative backup of all changes made since the last full backup. The advantage to this is the quicker recovery time, requiring only a full backup and the latest differential backup to restore the system. The disadvantage is that for each day elapsed since the last full backup, more data needs to be backed up, especially if a majority of the data has been changed.
Differential incremental backup 
Term for an incremental backup used by NetBackup.
Disaster recovery 
The process of recovering after a business disaster and restoring or recreating data. One of the main purposes of creating backups is to facilitate a successful disaster recovery. For maximum effectiveness, this process should be planned in advance and audited.
Disk image
A method of backing up a whole disk or filesystem in a single image. Since the underlying data structures are what is actually backed up, this method does not allow for file level control over what is selected for backup or restore.
Term for raw partition backup used by NetBackup Advanced Client. In NBAC, support is limited to the VxFS (Veritas), ufs (Solaris), Online JFS (HP-UX), and NTFS (Windows) filesystem types. Similar to the UNIX utility dump.
Full backup 
A backup of all (selected) files on the system. In contrast to a drive image, this does not included the file allocation tables, partition structure and boot sectors.
Hot backup 
A backup of a database that is still running, and so changes may be made to the data while it is being backed up. Some database engines keep a record of all entries changed, including the complete new value. This can be used to resolve changes made during the backup.
Incremental backup 
A backup that only contains the files that have changed since the most recent backup (either full or incremental). The advantage of this is quicker backup times, as only changed files need to be saved. The disadvantage is longer recovery times, as the latest full backup, and all incremental backups up to the date of data loss need to be restored.
Media spanning 
Sometimes a backup job is larger than a single destination storage medium. In this case, the job must be broken up into fragments that can be distributed across multiple storage media.
The practice of combining multiple backup data streams into a single stream that can be written to a single storage device. For example, backing up 4 PC's to a single tape drive at once.
The practice of creating multiple backup data streams from a single system to multiple storage devices. For example, backing up a single database to 4 tape drives at once.
Normal backup 
Term for full backup used by Windows Server 2003.
Near store 
Provisionally backing up data to a local staging backup device, possibly for later archival backup to a remote store device.
Open file backup 
Term for the ability to back up a file while it is in use by another application.
Remote store 
Backing up data to an offsite permanent backup facility, either directly from the live data source or else from an intermediate near store device.
Restore time 
The amount of time required to bring a desired data set back from the backup media.
Retention time 
The amount of time in which a given set of data will remain available for restore. Some backup products rely on daily copies of data and measure retention in terms of days. Others retain a number of copies of data changes regardless of the amount of time.
Site-to-site backup 
Backup, over the internet, to an offsite location under the user's control. Similar to remote backup except that the owner of the data maintains control of the storage location.
Synthetic backup 
Term used by NetBackup for a restorable backup image that is synthesized on the backup server from a previous full backup and all the incremental backups since then. It is equivalent to what a full backup would be if it were taken at the time of the last incremental backup.
Tape library 
A storage device which contains tape drives, slots to hold tape cartridges, a barcode reader to identify tape cartridges and an automated method for physically moving tapes within the device. These devices can store immense amounts of data.
True image restore 
Term used by NetBackup for the collection of file deletion and file movement records so that an accurate restore can be performed. For instance, consider a system that has a directory with 5 documents in it on Friday. On Saturday, the system gets a full backup that includes those 5 documents. On Monday, the owner of those documents deletes 2 of them and updates 1 of the 3 remaining. That updated document gets backed up as part of The Monday night incremental backup. On Tuesday afternoon the system crashes. If we perform a normal restore of the full backup from Saturday and the incremental backup from Monday to the fresh system, we will have restored the 2 documents that were intentionally deleted. True image restore keeps track of the deletions with each incremental backup and prevents the deleted files from being inappropriately restored.
Virtual Tape Library (VTL) 
A storage device that appears to be a tape library to backup software, but actually stores data by some other means. A VTL can be configured as a temporary storage location before data is actually sent to real tapes or it can be the final storage location itself.