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.
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.
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[edit] 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.
[edit] 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 recognised 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 periodical snapshots. 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.
[edit] 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 corrupt 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.
[edit] 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.
