How to use the network as a storage device

Network and server downtime is costing companies hundreds of thousands of dollars in business and productivity losses. At the same time, the amount of information to be managed and stored is increasing.

A new concept called the Storage Area Network (SAN) could offer an answer to the increasing amount of data that needs to be stored in an enterprise network environment. By implementing a SAN, users can offload storage traffic from daily network operations while establishing a direct connection between storage elements and servers.

Basically, a SAN is a specialized network that enables fast, reliable access among servers and external or independent storage resources. In a SAN, a storage device is not the exclusive property of any one server. Rather, storage devices are shared among all networked servers as peer resources. Just as a LAN can be used to connect clients to servers, a SAN can be used to connect servers to storage, servers to each other and storage to storage.

A SAN does not need to be a physically separate network, either. It can be a dedicated subnetwork, carrying only the business-critical I/O traffic between servers and storage devices. A SAN, for example, would not carry general-purpose traffic such as e-mail or other end-user applications. This type of net avoids the unacceptable trade-offs inherent in a single network for all applications, such as the need for dedicated storage devices for each server and burdening a LAN with storage and archival activity.

Building a SAN requires network technologies with high scalability, performance and reliability in order to marry the robustness and speed of a traditional storage environment with the connectivity of a network. As the SAN concept has developed, it has grown beyond identification with any one technology. In fact, just as LANs use a diverse mix of technologies, so can SANs. This mix can include FDDI, ATM and IBM's Serial Storage Architecture, as well as Fibre Channel. SAN architectures also allow for the use of a number of underlying protocols, including TCP/IP and variants of SCSI.

A SAN allows different kinds of storage - mainframe disk, tape and RAID - to be shared by different kinds of servers, such as Windows NT, Unix and OS/390. With this shared capacity, organizations can acquire, deploy and use storage devices more cost-effectively. SANs let users with heterogeneous storage platforms utilize all of its storage resources.

This means that within a SAN users can backup or archive data from different servers to the same storage system; allow stored information to be accessed by all servers; create and store a mirror image of data as it is created; and share data between different environments.

By externalizing storage and taking storage traffic off the operations network, companies gain a high-performance storage network, shared yet dedicated networks for the SAN and LAN, and improved network management. These features reduce network downtime and productivity losses while extending current storage resources.

In effect, the SAN does in a network environment what traditionally has been done in a back-end I/O environment between a server and its own private storage subsystem. The result is high speed, high fault tolerance and high reliability.

With a SAN, there is no need for a physically separate network because the SAN can function as a virtual subnet operating on a shared network infrastructure, provided that different priorities or classes of service are established. Fibre Channel and ATM allow for these different classes of service. Early implementations of SANs have been local or campus-based.

But as new WAN technologies such as ATM mature, and especially as class-of-service capabilities improve, the SAN can be extended over a much wider area.

Despite the hype about the coming of unlimited bandwidth, WAN services remain costly today. However, as WAN technologies improve their quality of service, they will provide the robustness needed for each application, including networked I/O, even over public WANs.

SAN tools

In addition to reliability and performance, SANs promise easier and less costly network administration. Today, administrative functions are labor-intensive and IS organizations typically have to replicate management tools across multiple server environments. With a SAN, there is just one set of tools and replication costs can be avoided. The traditional software functions of security management, access control, data management and storage management will be mapped into the SAN architecture and performed differently than they have been in the past. For example, different security strategies have to be pursued when storage devices are more widely available. Specialized I/O protocols such as Network Data Management Protocol are emerging, and the software functions will evolve much as LAN functionality has progressed in recent years. Kallhof is director of product marketing in the Enterprise Systems division of Computer Network Technology Corp. (CNT). CNT is a mainframe connectivity vendor, and also builds storage-area technology products. He can be reached at (612) 797-6000.