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Where to deploy CDP today

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Where does continuous backup fit in an overall storage strategy? Consider this scenario:
A data center with a centralized backup application, using some combination of tape and disk, provides the foundation for near-term data protection. This is supplemented by several other solutions. Microsoft Exchange e-mail is protected by an Exchange-specific CDP application that provides the ability to recover messages, mailboxes and information stores. It also provides point-in-time images of the information store for nightly backup. Key production database applications might be protected by a CDP appliance to provide instant recovery and also virtual images for backup and other purposes. File and print servers may be protected via snapshot software to provide appropriate levels of recoverability for that data.

Is CDP a good fit?
The thought of eliminating backup windows and providing nearly instant restores sounds great, but where does CDP fit within an overall strategy? Of course, not every product will fulfill all data production needs, and some storage environments may need to deploy a combination of products to achieve the preferred level of protection. (See "Major categories of data protection.")

To determine if CDP is an appropriate tool for your storage environment, some basic questions must be answered:

  • What problems can it solve today?
  • What tools or technology does it enhance or replace?
  • What is the value proposition?
To answer these questions, you must understand that continuous backup is not yet a replacement for traditional backup. Traditional backup applications move, manage and catalog data of all shapes and sizes from a large number of sources. These established products can maintain and store data for days, weeks, months or even years. While CDP products can assist with improving the backup process and can eliminate much of the dependence and pain of nightly backups, none of the products discussed have the breadth of capability and maturity to perform this fundamental data management function.

Also, most CDP products by themselves don't address disaster recovery, meaning they do not provide protection in the event of the loss of a site. To address this need, CDP can be supplemented either by replication or by traditional backup with off-site tape production. Many CDP vendors plan to add replication functionality to their products, but it's not available today.

At this time, the area that CDP best addresses is the backup and recovery of data associated with a particular set of applications. The characteristics of these applications are:

  • They usually run continuously
  • Their data changes frequently
  • Their data is stored in large containers making activities like backup difficult and time-consuming
  • There is a significant impact to the organization when they go down
These also happen to be the class of applications that traditional backup applications tend to handle poorly. The applications that meet these criteria in today's data centers are primarily e-mail and certain key applications that store information in databases.

Major categories of data protection

CDP provides many of the same benefits as split mirroring and snapshot technologies. Can it replace these solutions? In many cases the answer is yes, particularly if the requirement exists for improved RPO over what those technologies can practically provide. When evaluating a CDP solution vs. a disk mirroring or snapshot product, it's necessary to consider which features of each product are most important for your needs (see "Where to deploy CDP today"). Split-mirror and snapshot technologies--while often requiring some level of application integration--are for the most part application-agnostic, while a number of the CDP products are application specific. Also, technologies such as storage-based mirroring can be used by multiple servers accessing the storage system. Many of the CDP products are designed around protecting an individual host or application.

Ultimately, the question of value will determine if a CDP tool makes sense. The cost and benefits of the CDP products and their storage requirements must be evaluated against alternatives. Potentially, the most compelling cost comparison for CDP is against storage-based split-mirror solutions. In our original example of an environment that requires a four-hour RPO with the ability to retain 24 hours online, the disk requirement for a traditional split-mirror technology would be six times primary disk or greater, depending on the level of data protection required for those disks. The CDP disk requirement would be approximately one and a half to two times the primary storage, depending on rate of change (number of write transactions) and level of protection. While this is a substantial difference, it should be further noted that most split-mirror solutions require the use of the same high-cost storage as the primary volumes. CDP can, if desired, take advantage of lower-cost secondary disk storage.

Continuous data protection will continue to evolve to provide stronger integration with data intensive applications. As replication technologies are adopted or built into products, CDP solutions should be able to provide robust and cost-effective disaster recovery scenarios, as well. Who knows? Maybe someday we'll look back at the idea of nightly backups, scratch our heads and wonder why anyone would ever have done that.

This was first published in June 2004

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