 implement virtualization on their own storage devices, whereas oth- ers provide storage virtualization on a variety of devices. However, currently, no single vendor pro- vides across-the-board virtualization for any indiscriminate choice of storage device. Storage virtualization can be implemented in three different ways: On a host CPU or server. On a storage array. On an appliance. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STORAGE NETWORK PROCESSORS (SNPS) STORAGE NETWORK PROCESSORS (SNPs) 201 Another approach vendors take to attain the virtualization objective is to implement the following within one of these two categories: Symmetric storage virtualization (also known as in-band storage virtualization). Asymmetric storage virtualization (also known as out-of-band storage virtualization). These names are derived from the fact that in the in-band approach, a device lies in the actual path of data that must be exchanged between a server and devices. It passes data and/or intelligence through to arrays that are attached to it. Conversely, in the out-of-band approach, data is passed between a server, switch, or router to the devices. The entire work is managed by the server or storage array. If we take a closer look at the three fundamental platforms of implementing storage virtualization, we will notice some interesting characteristics: Server- or array-based virtualization was the first way to implement this technology. In this scheme, both the storage and the data-pooling intelligence reside on the server or array. Because this approach does not put any other devices on the path that the data must traverse, it scales better than network-based virtualization. When all virtualization work must be done on the server, no other devices on the network, such as Fibre Channel switches or other arrays, are affected. The downside of the approa