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GCN Home > November 5, 2001 issue

A SAN with a plan

By Kevin Jonah, Special to GCN

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Fibre Channel devices put you in touch with large stores of off-site data but don’t impact your apps

Data is the grist of any application. The trick has always been getting that data from where it is to where it needs to be without causing bottlenecks in application performance. Today, with the mushrooming storage requirements of distributed applications, managing performance is one of the biggest headaches for system administrators.

High-reliability storage was once limited to mainframe and high-end server systems. But the expanded use of clusters of commodity-level servers and operating systems such as Microsoft Windows NT and 2000 for essential applications has made manageable, redundant and affordable storage solutions a necessity for many organizations.

Now, with recent events reawakening interest in technologies such as off-site mirroring of applications for disaster recovery, the storage management headache has grown more severe. Maintaining dedicated storage for every Web server or application server can become expensive, while sharing disk space over LAN connections can cause major application bottlenecks.

Get connected

The way around the problem is with a storage area network. These dedicated networks connect servers, or clusters of servers, to pools of data storage and archiving devices at higher transfer rates and over greater distances than are practical with conventional storage connection technologies. SANs provide an infrastructure for building redundant, high-availability storage for groups of servers, and they give system administrators a way to expand or upgrade the storage available to servers without having an impact on running applications.

The leading infrastructure technology for SANs is Fibre Channel, which offers up to 2 Gbps of throughput—although the throughput for any single device is limited to 100 Mbps, or 200 Mbps in a full-duplex configuration. Fibre Channel can connect devices at distances up to six miles if the connection is over fiber-optic cabling.

New technologies

A 4-Gbps version of Fibre Channel currently is under development. But Fibre Channel is facing a challenge from other technologies that promise greater transmission speeds, lower costs or both. The technology was devised for point-to-point connections between computer systems and storage devices needing a faster, more robust peripheral link than SCSI could offer.

Then came a loop configuration—Fibre Channel-Arbitrated Loop, or FC-AL—which offered a way to chain multiple devices together. The latest evolution is a switched fabric, with a high-speed switch creating virtual direct connections between devices as required.

A number of data transfer protocols can be used over Fibre Channel connections, including SCSI and IP. This flexibility lets Fibre Channel support multiple storage applications.

One way Fibre Channel technology takes advantage of the flexibility is by tunneling over IP networks—including Ethernet. Tunneling can be especially useful for mirroring data from site to site over a metropolitan network or WAN, either in support of distributed applications or for replication to a backup hot site for disaster recovery or off-site redundancy.

Using fiber-optic IP networks to carry data effectively extends the six-mile limitation on Fibre Channel connections even further. Backup tape systems connected by Fibre Channel-over-IP can store data offline at a secure, remote location. Fibre Channel SANs also can work over longer distances via other network architectures, including asynchronous transfer mode and Dense Wavelength Division Multiplexing.

Compatibility issues

Fibre Channel infrastructure products fall into three categories: host bus adapters, hubs and switches. Hubs offer small-scale SAN capabilities based on the FC-AL loop architecture. Switches create larger, higher-throughput SANs and are generally much more expensive. Host bus adapters connect servers and network hardware to the Fibre Channel SAN.

The main problem with Fibre Channel has been compatibility. Fibre Channel standards were loosely adhered to in the past, or extended in proprietary ways, so the result has often been conflicts in hardware from different vendors. Manufacturers recently have worked out a lot of the incompatibilty issues, but now other technologies are posing a challenge to Fibre Channel—most of them from outside the storage-specific networking world.

One of the challengers is Gigabit Ethernet. With 10-Gigabit Ethernet close to becoming a reality, Fibre Channel’s throughput edge for SAN applications is rapidly evaporating.

And with faster, less-expensive technology becoming available for short-distance connections to storage devices, other methods will squeeze Fibre Channel on the point-to-point front. For example, the Serial ATA interface standard, released in August but not yet widely adopted by hardware vendors, supports throughput of 1.5 Gbps and a cable length of up to a meter.

Another challenge of sorts for SAN technology is network-attached storage [GCN, Oct. 22, Page 32]. NAS devices are essentially specialized file servers that provide a large amount of network-mountable disk storage to both clients and servers on a LAN. They generally rely on Ethernet for connectivity, and usually support Microsoft Windows and standard network file-sharing protocols such as the Network File System.

NAS or SAN?

In the past, NAS technology offered an easy way to mount more shared disks on a LAN. But with the increasing speed of Ethernet, NAS devices—when combined with high-speed Ethernet switched networks—could challenge current SAN technology. And NAS devices often can be used as a complement to SANs—or in some cases, as an access point to them—because many NAS units rely on Fibre Channel connections to the storage devices themselves.

The main advantage of Fibre Channel SANs in the face of the growing list of alternatives is their lead in support for tools such as storage management software. Fibre Channel SAN switches such as Gadzoox Networks Inc.’s Slingshot series already are certified for enterprise management software including Computer Associates International Inc.’s Unicenter and Tivoli Systems Inc.’s Storage Network Manager module.

SAN switches also support a host of other standard network management tools and interfaces. But in the end, the one thing that can keep Fibre Channel competitive is its decreasing price. As vendors drive down the price per port of Fibre Channel switches, Fibre Channel could still be the SAN of the future for many organizations.

Kevin Jonah, a Maryland network manager, writes about computer technology.

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