Continuous data protection
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Continuous data protection (CDP), also called continuous backup or real-time backup, refers to backup of computer data by automatically saving a copy of every change made to that data, essentially capturing every version of the data that the user saves. It allows the user or administrator to restore data to any point in time. The technique was invented by British entrepreneur Pete Malcolm in 1989.
CDP is a service that captures changes to data to a separate storage location. There are multiple methods for capturing the continuous changes involving different technologies that serve different needs. CDP-based solutions can provide fine granularities of restorable objects ranging from crash-consistent images to logical objects such as files, mail boxes, messages, and database files and logs.
Differences from traditional backup 
Continuous data protection is different from traditional backup in that you don't have to specify the point in time to which you would like to recover until you are ready to perform a restore. Traditional backups can only restore data to the point at which the backup was taken. With continuous data protection, there are no backup schedules. When data is written to disk, it is also asynchronously written to a second location, usually another computer over the network. This introduces some overhead to disk-write operations but eliminates the need for scheduled backups.
Continuous vs near continuous 
Some solutions which are marketed as continuous data protection may only allow restores at fixed intervals such as 1 hour, or 24 hours. Such schemes are not universally recognized as true continuous data protection, as they do not provide the ability to restore to any point in time. Such solutions are often based on periodic snapshots, an example of which is CDP Server, disk-based backup software that periodically creates restore points using a snapshot and volume filter device driver to track disk changes. There is some debate in the industry as to whether the granularity of backup needs to be "every write" in order to be considered CDP or whether a solution which captures the data every few seconds is good enough. The latter is sometimes called near continuous backup. The debate hinges on the use of the term continuous: whether only the backup process needs to be continuous, which is sufficient to achieve the benefits cited above, or whether the ability to restore from the backup also has to be continuous. The Storage Networking Industry Association (SNIA) uses the "every write" definition.
Differences from RAID/replication/mirroring 
Continuous data protection differs from RAID, replication, or mirroring in that these technologies only protect one—the most recent—copy of the data. If data becomes corrupted in a way that is not immediately detected, these technologies will simply protect the corrupted data.
Continuous data protection will protect against some effects of data corruption by allowing to restore a previous, uncorrupted version of the data. Transactions that took place between the corrupting event and the restoration will be lost, however. They could be recovered through other means, such as journaling.
Backup disk size 
In some situations, continuous data protection will require less space on backup media (usually disk) than traditional backup. Most continuous data protection solutions save byte or block-level differences rather than file-level differences. This means that if you change one byte of a 100 GB file, only the changed byte or block is backed up. Traditional incremental and differential backups make copies of entire files.
Risks and disadvantages 
The protection afforded by continuous data protection is often heralded without consideration of the disadvantages and challenges that it can present. Specifically, the continuous bandwidth usage can adversely affect network performance, especially in operations where file sizes are large, such as multimedia and CAD design environments. To mitigate this risk, companies employ throttling techniques which prioritize network traffic in order to reduce the impact of backup on day-to-day operation.
See also 
- Disaster recovery
- Single instance storage
- List of backup software
- List of online backup services
- InMage DR-Scout
- R1Soft Continuous Data Protection
- EMC RecoverPoint
- Cofio Software
- near-Continuous vs. True CDP
- Categories of Backup Software
- CDP is a Dog - Unless its Unified with Backup
- Continuous Data Protection for programmers An article on how CDP can help programmers.
- Behtash, Behzad (2010-05-10). "Why Continuous Data Protection's Getting More Practical - Services - Disaster recovery/business continuity". Informationweek. Retrieved 2011-11-12.
- US Patent 5086502 “A method of operating a data processing system” filed Nov 13, 1989.
- "An Overview of Continuous Data Protection". Infosectoday.com. Retrieved 2011-11-12.
- Off-Site Backup - The Bandwidth Hog