Ultra2 SCSI. Today's Ultra Wide SCSI interface presents several problems to server designers. The most serious problem is performance: 40MBps may seem like a lot, but four modern disks running at full speed can still saturate it. A less widely understood problem is that every time SCSI speed has doubled, the average total length of the cabling from controller to disk has been cut in half. In addition, although you can put several SCSI channels on one PCI board, because of the size of the connectors, you can't easily offer more than two channels per board to external devices. These restrictions in cable length and slot availability make adding large amounts of storage to servers difficult.
Ultra2 SCSI (also known as Low Voltage Differential Signaling--LVDS--SCSI) addresses both of these limitations. First, Ultra2 increases bandwidth to 80MBps. Second, because Ultra2 uses differential signaling (+5 and 5 volts rather than +5 and 0 volts), it allows the use of longer cables than current SCSI implementations allow. Ultra2 SCSI is available today; unfortunately, it's not widespread. You can learn more about Ultra2 technology at http://www.adaptec.com/technology/whitepapers/futureofscsi.html.
Fibre channel storage subsystems. Fibre channel fundamentally changes the way we connect devices to computers. It does so by using a high-speed serial interface instead of the parallel interface SCSI uses. (A serial bus can be faster than a parallel bus because driving a serial bus at very high frequencies is much easier.) In their initial implementations, fibre channel storage subsystems will enable transfer speeds of 100MBps; fibre channel's future upgrade path offers speeds of up to 400MBps. This serial transmission mode addresses the problem of signal degradation in long cables, allowing the use of cables whose lengths are measured in kilometers. The name fibre channel is somewhat misleading, because it implies the use of fiber optics cables. Actually, many fibre channel implementations will use copper wiring, which is cheaper than fiber-optic cabling and still allows very long (hundreds of meters) cable lengths. Longer cables let you locate a server in a building separate from the server's storage, a particularly useful feature in clustered environments in which separating a server from its storage facilitates disaster recovery and heightens security.
Fibre channel supports as many as 126 devices per channel. However, users must still choose between performance and expansion, because peak performance will occur with around 30 devices to 40 devices on a single controller. You can implement fibre channel with a great deal of fault tolerance by running dual-fibre loops from one disk to two systems, each with its own disk controller, to provide redundancy, as Figure 1 shows. You can find out more about fibre channel by visiting the Fibre Channel Association Web site (http://www.fibrechannel.com). To read a preview of fibre channel performance, see Dean Porter, "Fibre Channel, SCSI, and You," September 1997.
PCI Advances
At PCI's introduction in 1994, it represented an advance over EISA and ISA bus technologies, which offered a peak bandwidth of 133MBps. Today, the PCI bus looks increasingly like a serious bottleneck as the computer industry looks forward to advances like fibre channel (100MBps) and Gigabit Ethernet (80MBps to 100MBps). Fortunately, the designers of PCI anticipated the problem and designed PCI to be scalable. The next generation of servers will offer a 64-bit PCI bus capable of 267MBps peak bandwidth when running at 33MHz (current PCI bus implementations are 32-bit at 33MHz).
Speed isn't the only advance coming to the PCI bus. Already several vendors have offered hot-plug PCI slots in their systems. With hot-plug, you can replace a failed PCI card without a system reboot. Hot-plug PCI combined with fault-tolerant technology such as fibre channel means you'll never again have to say you're sorry because your system is down.
High-Speed Dedicated Clustering Interconnects
The current 2-way failover cluster that NT Server, Enterprise Edition (NTS/E) supports is rather primitive compared with more mature clustered environments, such as Digital Equipment's VMS clustering, that support load balancing across many more systems in a cluster. Microsoft plans to change this situation with a future release of NTS/E that supports not only failover but also load balancing between cluster members. However, when you begin load balancing between clusters, you generate housekeeping traffic between the servers in the cluster.
In theory, Microsoft could use a standard protocol such as TCP/IP to implement failover and load balancing over a conventional network such as 100Base-T. However that approach introduces a problem with network latency. Current design lets networks and networking protocols handle general-purpose networking, making acceptable a delay of hundreds of milliseconds between the sending of a packet and acknowledgment of its receipt. However, in intracluster communications, latency would prevent a 2-node cluster from scaling effectively. Adding nodes to the cluster would increase data-relay time with each additional node, eventually affecting performance unacceptably. (A high-speed interconnect that can overcome the data-relay bottleneck would consist of a software layer--an OS--and a hardware component. Microsoft will likely base such a software component in NT 5.0 on the Virtual Interface--VI--architecture it is developing jointly with Intel and Compaq Computers. You can find the technical details describing VI on Intel's Web sitehttp://www.intel.com/solutions/tech/via.htm.)
Several companies are working on dedicated high-speed clustering interconnect technologies. Compaq and Tandem collaborated on ServerNet, which Tandem originally developed for its proprietary Himalaya systems. ServerNet is now available to other vendors as an NT cluster interconnect solution (http://www.tandem.com). Dolphin Interconnect Solutions has designed an interconnect for UNIX clustering solutions that implements the Scalable Coherent Interface (SCI) standard. This product is now available to vendors as an NT solution (http://www.dolphinics.com/dolphin2/interconnect). HAL Computer Systems recently announced its entry into the high-speed interconnect marketplace with its Synfinity Interconnect Architecture. This product addresses a variety of clustering interconnect needs, including VIA and CC-NUMA (http://hal.com/fjst/). All these cluster interconnect solutions share several features, including performance in the GB-per-second range combined with very low latency. This feature set means cluster members can communicate with one another at very high speeds, allowing them to scale efficiently as new members join the cluster.
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