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Allocation efficiency is only the first step toward realizing an even more important goal of storage utilization efficiency. Utilization efficiency refers to the cost-effective distribution of data across storage platforms, based on different capabilities and price tags of storage platforms and the characteristics of the data itself. Utilization efficiency is required to ensure you get the most value from your storage investment while providing the right kind of storage for the data that occupy it.

The ENSA SAN promised utilization efficiency as a function of an intelligent storage pool. The storage pool would sense, in effect, the needs of the application and allocate not only the right capacity but also the right "flavor" of storage. A streaming multimedia application, for example, would be allocated the outermost tracks of disk platters because this was the longest contiguous storage space on the disk and would facilitate jitter-free playback of stored files. Similarly, a database with 15 elements would be allocated storage with a RAID level appropriate to each element—RAID 5 for some elements, RAID 3 for others, and RAID 0 (no RAID) for the rest—in order to optimize application performance.

What the ENSA pioneers were underscoring was a simple truism: It was not enough to simply provide an application with storage. Applications had specific requirements for the types of storage that they needed. This key point continues to elude many of those who portray SANs as simply "plumbing."

Capacity utilization efficiency goes beyond application requirements to include also the lifecycle requirements for data and practical considerations, such as platform cost. For example, many organizations store data to a high-end storage platform where its frequency of access or reference drops by 50 percent within three days, and by 90 percent within a month. Like it or not, the preponderance of data, including e-mail, fits this description in many shops. This characteristic or pattern of data utilization demonstrates how inefficiently we utilize storage capacity by allocating our most expensive storage to store data that isn't being referenced very often.

Clearly, it would be smarter to offload less-frequently referenced data either to a tier of inexpensive IDE/ATA storage arrays or to archival tape. Contemporary storage networking technology enables the interconnection of these devices into multitiered storage configurations. Lacking is the intelligence to do much with them.

Moore and others observe that, even if capacity allocation efficiency were somehow to be optimized, the cost efficiency of the storage infrastructure in most shops would continue to hover at around only 30 to 35 percent.[2] A combination of allocation and utilization efficiency is required to drive down storage costs effectively. Contemporary SANs deliver neither and therefore fail also to deliver on their promise of cost-efficient hardware utilization.

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