How to cook up a storage-area network
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Storage-area networks (SAN) are being adopted more frequently today, but uncertainty still exists among users about why to install them and the applications that can make the best use of them. SANs are high-speed networks, similar to LANs, that connect disk subsystems directly to servers or clients. The idea is to relieve network congestion or bypass distance limitations imposed by traditional SCSI storage connections.
Fibre Channel-based SANs can be configured in several ways and for any number of reasons (see graphic). They can be installed as Fibre Channel Arbitrated Loops (FCAL) or switched fabric networks. They can be local or remote, spanning campuses and using wide-area connections.
Methods for implementing SANs may differ, but most network professionals offer the same recipe from one installation to another: start small and think globally. They recommend implementing small, simple FCAL or switched fabric networks to share data among servers.
"You need to approach SANs with a broader plan than simply dropping in [more storage] and seeing if it is going to be successful,"says Lee Congdon, senior vice president of strategic integration and architecture for NASD in New York. NASD is the parent company of the NASDAQ stock exchange.
"You need to plan not only where you are today, but where you want to be tomorrow and then consciously choose the applications that work in the context of a bigger picture," Congdon adds.
Look to your apps
Congdon and other users say to look at your applications to determine which applications warrant adding a SAN and which will do just as well on server-attached RAID. They say to begin with a small, low-impact or low-budget application, such as consolidating several servers to the same storage device, before moving to full-blown implementation. From this application, users can test the "proof of concept," and then grow the SAN to involve more of the routine tasks of the data infrastructure. For example, data replication and creating multiple active copies of data that would increase utilization or ease maintenance and testing operations.
Users also recommend dividing applications into categories based on importance. For mission-critical application data, users need to design in redundancy and fault tolerance with duplicated storage, switches and host bus adapters, so there is no single point of failure.
For example, at NASD, Congdon has several Sequent NUMA servers running Unix connected via a Fibre Channel switch to a single EMC Symmetrix disk subsystem; Dwight Gibbs, chief technical fool at The Motley Fool, connects Windows NT servers via a hub to a Compaq StorageWorks disk array; and Bruce Covey of Home Depot in Smyrna, Ga., connects numerous Macintosh workstations to Avid video editing storage via a Fibre Channel hub.
Prepare for future
Not surprisingly, users have implemented SANs in different ways, but most are preparing for future growth.
For example, Congdon's switched fabric SAN will be able to easily expand as he adds more applications. The Motley Fool and Home Depot use FCAL and will need to migrate to a switched fabric architecture before they can add any more servers, clients or applications to their SANs.
Users agree, though, that relying on a systems vendor or integrator for your first SAN implementation is the wise route.
Gibbs says you should also expect your vendor to guarantee interoperability between Fibre Channel and SCSI devices.
"I want a 60- to 90-day money-back guarantee. If it works, I'll keep it. If not, I want to return it. So far, I have not run into a situation where we just couldn't make something work," Gibbs says.
While Congdon generally agrees with Gibbs, he takes vendor participation a step further. He thinks that it is reasonable to choose a vendor to implement a limited package. He adds that it is unlikely a vendor would be able to completely understand business applications and where a midsize to large business is going.
Congdon and Gibbs also believe the plan for a data infrastructure closely parallels the network and systems infrastructure. Blending the three into a single cohesive network is necessary to the planning. That way, storage becomes another infrastructure resource that users can manage. This strategy also helps ensure network, applications and other groups in the enterprise network environment interoperate.
Successful SAN implementation also depends in large part on getting the approval of management, says Steve Rigney, a consultant with NetReference in Sterling, Va.
For example, Congdon's SAN is just the start of a much larger SAN implementation that management has approved and provided the funds for. He wouldn't have the SAN if he hadn't convinced people of the cost savings and benefits of centralized storage solutions over stovepipe storage growth.
"Ensuring that you get a properly funded program and a disciplined investment plan rather than budgeting in fits and starts is important," Congdon says. "You don't want the first application to have to bear all the cost. You need to think about planning for this, investing in it, and then allocating the cost back to the business across multiple applications."
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