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How does content-addressed storage differ from backup?

The term content-addressed storage (CAS) refers to systems that store data in such a way that an object cannot be duplicated or modified once it has been stored. Different vendors use various terms to describe these

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types of products. The Evaluator Group uses the term "Fixed Content and Reference Storage (FCR)" to describe this class of storage. In contrast, a backup is a duplicate copy of the original, stored on separate media.

If there is only one copy of data, and it is moved to slower, less costly storage, this is known as an archive. One of the original purposes for FCR storage devices was to allow users and applications to access archival data more quickly than was often available with data residing on tape media.

For random access, FCR storage is faster than tape, but slower than primary storage. As a result, the cost for FCR storage is somewhere between traditional disk storage and tape storage, representing a cost and performance point between these options.

CAS does not relate directly to backups; they are two different concepts. That is, fixed content, reference or CAS is not a backup if it is the only copy.

However, like all other important information, data stored on an FCR device should be backed up or otherwise protected. Thus, most storage known as FCR or CAS allows the use of some type of data protection method, such as copying to tape, or copying to another FCR unit in a remote location.

FCR storage devices are designed to store data at a lower cost than traditional disk arrays, with better random access performance than tape storage. Additionally, FCR storage typically allows access at a file or object level, whereas backup and restore operations typically operate at the device block level.

While the performance of FCR storage is sufficient for its intended use, these products are not designed for high-speed sequential access. Sequential access is the type of data transfer that occurs with backup operations. For all the reasons outlined, FCR devices are not designed for use as a backup target.

Backing up data should always occur. The choice to deploy FCR storage as a tier of storage between disk and tape and to provide regulatory compliance or object access to data, is a separate and distinct choice.

About the author: Russ Fellows is a Senior Analyst with Evaluator Group. He is responsible for leading research and analysis of product and market trends for NAS, virtual tape libraries and storage security. He is also the primary analyst for coverage of selected open-system arrays and virtualization products. Russ is a well regarded and successful industry professional with 20 years of high technology experience, including product design, product development, systems engineering, business strategy development, competitive analysis and portfolio management within both the vendor and end-user groups.

This was first published in July 2008

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