Continuous data protection

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. In its true form it allows the user or administrator to restore data to any point in time. The technique was patented by British entrepreneur Pete Malcolm in 1989 as "a backup system in which a copy [editor's emphasis] of every change made to a storage medium is recorded as the change occurs [editor's emphasis]" .

CDP runs as a service that captures changes to data to a separate storage location. There are multiple methods for capturing continuous live data 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.

There is a briefer sub-sub-section in the "Backup" article about this, now renamed to "CDP" to avoid confusion.

Differences from traditional backup

True continuous data protection is different from traditional backup in that it is not necessary to specify the point in time to recover from until ready to restore. Traditional backups only restore data from the time the backup was made. True continuous data protection, in contrast to "snapshots", has 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

Because true CDP "backup write operations are executed at the level of the basic input/output system (BIOS) of the microcomputer in such a manner that normal use of the computer is unaffected", true CDP backup must in practice be run in conjunction with a virtual machine—which rules it out for ordinary personal backup applications. It is therefore discussed in the "Enterprise client-server backup" article, rather than in the "Backup" article.

Some solutions marketed as continuous data protection may only allow restores at fixed intervals such as 15 minutes or one hour or 24 hours, because they automatically take incremental backups at those intervals. Such "near-CDP" schemes are not universally recognized as true continuous data protection, as they do not provide the ability to restore to any point in time. When the interval is shorter than one hour, these "snapshot" solutions are typically based on periodic snapshots, an example of which is Arq Backup, disk-based backup software that periodically creates restore points using a snapshot. "Near-CDP" solutions use snapshots because "to avoid downtime, high-availability systems may instead perform the backup on ... a read-only copy of the data set frozen at a point in time—and allow applications to continue writing to their data".

There is debate in the industry as to whether the granularity of backup must be "every write" to be CDP, or whether a "near-CDP" solution that captures the data every few minutes 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 must be continuously automatically scheduled, which is often sufficient to achieve the benefits cited above, or whether the ability to restore from the backup also must be continuous. The Storage Networking Industry Association (SNIA) uses the "every write" definition.

Differences from RAID, replication or mirroring

Continuous data protection differs from RAID, replication, or mirroring in that these technologies only protect one copy of the data (the most recent). If data becomes corrupted in a way that is not immediately detected, these technologies simply protect the corrupted data with no way to restore an uncorrupted version.

Continuous data protection protects against some effects of data corruption by allowing restoration of a previous, uncorrupted version of the data. Transactions that took place between the corrupting event and the restoration are lost, however. They could be recovered through other means, such as journaling.

Backup disk size

In some situations, continuous data protection requires 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 one byte of a 100 GB file is modified, only the changed byte or block is backed up. Traditional incremental and differential backups make copies of entire files; however starting around 2013 enterprise client-server backup applications have implemented a capability for block-level incremental backup, designed for large files such as databases.

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 that prioritize network traffic to reduce the impact of backup on day-to-day operation.

See also

• Quest AppAssure
• Cofio Software
• Disaster recovery
• EMC RecoverPoint
• FalconStor
• InMage DR-Scout
• List of backup software
• List of online backup services
• R1Soft Continuous Data Protection
• Single instance storage
• CloudEndure