Like money, data has value in time. This is an important consideration when working out the most cost-effective strategy for archiving it on your SAN. It also means that the standards for archiving data (long- term storage), backing up data, and merely storing data for use, can be very different.
In general, data loses value to the enterprise with time, but different kinds of data lose it at different rates. Transactional data is typically business critical when it is generated but it declines in importance rapidly and usually doesn't need to be highly available from long-term storage. In contrast, a data-mining database usually doesn't have to be restored as quickly as transaction data, but it tends to have a much longer useful life span and often needs to be available quickly when archived. As a result transactional data should be backed up so it can be restored most quickly (and expensively) but it can be archived using methods like off-line tape, which are cheaper but have slower access. How much and what kind of storage you'll need in your SAN are important questions that can hinge on considerations such as these.
Time value of data really depends on the answers to two questions: How often will this data be accessed once it is archived and how important is fast access to this data? Once you have the answers to those questions, you can decide what kind of archival storage is appropriate for the data. By tailoring the method of archiving to the time value of the data you can reduce storage costs while maintaining an appropriate level of service.
According to Sun Microsystems, in deciding on an archiving strategy, administrators should balance the cost of the method of storage against the speed of access using the time value of the data as a standard. Data with a high value over time may be kept on disk, for example, while seldom-accessed data could be shifted to a tape library, or even off-line tape.
Sun Microsystems discusses this issue in an article titled "Balancing Quality of Storage Service with Total Cost of Ownership" available at its Web site at www.sun.com.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.