Building a SAN to service two worlds


Can you build a high-performance storage area network (SAN) with tried-and-true SCSI? Yes. Can the same SCSI-based SAN take advantage of Fibre Channel, while reducing some its limitations? Yes. In this article, I'll tell you how.

Creating an Ultra 160 SAN

"To build a SAN with Ultra 160 SCSI, use a star topology with the storage in the middle of the star."

Ultra 160 SCSI, the current version of this interconnect, is backward compatible with other versions of SCSI. It provides a 68-pin bus with 160MB per port between the computer and the storage. Single PCI cards come with two of these ports. Combined, you can reach up to 320MBps. To achieve this speed, you need a PC with a 64-bit PCI bus running at 66MHz. Some of the newest Macs and Windows NT servers offer this bus, as well as a 100MHz bus; Sun Microsystems servers have always had this bus.

Ultra 160 SCSI provides very reliable, point-to-point connectivity, which in turn allows data to get on and off disks very quickly. However, it does have a distance limitation of 25 meters from the computer to the host or from the computer to the storage system.

To build a SAN with Ultra 160 SCSI, use a star topology with the storage in the middle of the star. A cable goes from the star to the storage to each of the servers. This technology currently supports four to six servers sharing the same storage, not 126 nodes like Fibre Channel. However, four to six servers can provide a powerful backbone to the network. The benefits include massively parallel connectivity--you have not one SCSI cable to your storage, but four or eight. (Vendors offering these cards include Adaptec Inc., LSI Logic Corp., Q-logic Corp., and others.) Eight ports with only four PCI cards provide more than 1GBps bandwidth. Even better, the set up doesn't take too long.

In a mixed SCSI-Fibre Channel SAN, you can have multiple RAID systems, each with up to 400MB. (See Figure 1) (This solution can go up to 4Tb.) This layout also shows a single Ultra 160 SCSI low-voltage differential (LVD) bus adapter, which has two ports: bus 1 and bus 2. Each port goes to each server. In turn, each server connects to the SAN via either Fast Ethernet or Gigabit Ethernet.

Mixed SCSI-Fibre Channel SAN
Figure 1: A mixed SCSI-Fibre Channel SAN showing three servers and two RAID systems. (Source: Seybold 2000)

With middleware software, such as Tivoli's SANergy version 2.0, any server in the system can fail and the entire system will keep running. SANergy allows the other servers in the network direct access to the storage. SANergy performs file locking and manages those files. The file handshake transaction that determines who has the file comes over the TCP/IP Ethernet. The data can be transmitted via Ultra 160 SCSI at speeds three to 20 times bandwidth over what TCP/IP usually provides. This speed results from the servers having direct access to the file area without having to transfer data over the network.

A mixed SCSI-Fibre Channel SAN

"With middleware software any server in the system can fail and the entire system will keep running."

Fibre Channel, an emerging technology, consists of a technology converted from SCSI to optical to a very specialized packet. Fibre Channel does two things: It runs the SCSI protocol at 100MB per port over optical cables, and it runs a unique storage protocol at 1.06Gbps in packets. (Fibre Channel does not currently run IP.) It's really SCSI using a different protocol. As a network topology, Fibre Channel uses a hub or a switch as a concentrator. The switch runs faster than the hub. Fibre Channel supports up to 500 meters, which is suitable for most applications. (You can spend more money and purchase special cables and drivers to go up to 10 kilometers.)

Current Fibre Channel Arbitrated Loop (FC-AL) has one downside: it runs Class 3 service. Three classes exist for quality of service of transmission, and Class 3 service doesn't guarantee or acknowledge transmission. If a fibre drops a packet and the software fails to catch it, the result is a hang up (or a time out), causing the system to momentarily freeze. The reinitialization loop process begins, resetting the entire bus.

Because SCSI is point-to-point, it doesn't have the benefits of running the loop. You pay more for the benefits of Fibre Channel over Ultra 160 SCSI, but Fibre Channel can also go a greater distance than SCSI.

Building a SAN with Fibre Channel differs from using SCSI. You still have a star topology like a network, with a hub and switch in the center. The bandwidth of the hub is 100MB and the bandwidth of the switch is higher. Although you can have 126 nodes on this star-arbitrated loop, you could have difficulty managing and debugging that many nodes-so you may want to limit the nodes to 12 or 14.

The layout in Figure 2 shows the three servers running directly to the RAID system. In addition, an inexpensive fibre-to-SCSI bridge converts traditional SCSI to Fibre Channel SCSI. You can now include a hub or a switch attached to a workstation and/or the servers. Why would you do this? SCSI doesn't require a concentrator, a hub, or a Class 3 service. You could dedicate these three servers to certain functions. For example, your Mac and NT power users or remote users may need the faster access to data without clogging the network. They can connect via Fibre Channel to a Fibre Channel hub or switch, which in turn connects to a FC-AL to a SCSI bridge to the storage. (Each fibre, hub, or switch has fewer than 126 nodes.)

Fibre Channel SCSI
Figure 2: The ultra 160 SAN layout lets servers connect directly to the RAID system and allows hubs or switches to be included in the network architecture. (Source: Seybold 2000)

Again, Tivoli's SANergy makes this setup work by providing the servers with simultaneous access to the storage. Installing SANergy on the Macs and NT servers gives them transparent access to the data. If anything happens to the optical transmission or to the switch, SANergy automatically reroutes the traffic back over the LAN. So, if a particular network hangs up or if a connection goes bad, the SCSI won't fail. If it does fail, then typically the servers will take over.

SCSI provides a simple, reliable, fast connection right to the storage in the data center. Meanwhile, Fibre Channel provides connectivity for a large number of users, and allows for greater distance. In the case of a failure, SANergy allows employees to continue running (not necessarily at optimal speed) until you have a chance to debug or figure out the problem. The end result includes the best of all worlds. //

Jerry Namery, president and CTO of Winchester Systems Inc., oversees the company's research, development, and manufacturing operations. Namery was the invited to speak at Seybold 2000, the printing and publishing industry's largest trade show and conference. Namery received both a BS in Electrical Engineering and an MS in Electrical Engineering from Massachusetts Institute of Technology.


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