Feature

Directors take on more tasks

In the early days of storage networking, Fibre Channel (FC) switches with four to 32 ports served the needs of most organizations. When requirements scaled beyond 32 ports, switches were connected using inter-switch links (ISLs). This architecture gradually evolved into a so-called "core-to-edge" fabric, with larger switches in the "core" and smaller switches at the "edge" of the storage infrastructure. Data directors were reserved for "monolithic" implementations, usually focused on the mainframe.

Indeed, it's the mainframe environment from which data directors (also called channel directors) evolved. In the open-systems world, these products are called director-class switches, but the architecture is the same: More than 128 ports in a single package (frame) designed to offer high, predictable performance and at least 99.999% system availability. The core-to-edge architecture offers the advantages of incremental growth, but becomes problematic when ISLs become too numerous and latency becomes a problem. However, director-class products cost 25% to 50% more per port to implement initially because of the base cost of the frame and additional redundant hardware features. For IT organizations considering directors for the first time, or those re-examining their fabric strategy, three questions must be answered:

  • Under what circumstances does it make sense to use core/edge switches, and under what circumstances should directors be used?
  • Does it make sense

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  • to implement a heterogeneous fabric to get the "best of all worlds"?
  • What are the key technology differentiators that should be considered between vendors?

Differentiating directors
Several key differences distinguish directors from switches. The first criterion is the number of ports. Although some 64-port devices are referred to as directors, most directors start at 128 ports. Second, directors have "non-blocking" architectures, meaning that all ports can operate at full speed simultaneously. In contrast, the internal communications of a switch are "oversubscribed" when the aggregate speed of the ports exceeds the internal bandwidth of the device. Third, directors should be capable of non-disruptive upgrades, including firmware updates. If a device must be taken offline for an upgrade, it can't deliver the 99.999% online objective of most directors. The impact of taking hundreds of ports offline for an upgrade is unacceptable to many organizations. Finally, a director should be able to isolate faults to a specific zone without impacting ports in other zones.

The competition among director-class products has never been more intense than it is now. Presently, four vendors compete in the director market: Brocade Communications Systems Inc., Cisco Systems Inc., Computer Network Technology (CNT) Corp. and McData Corp. (McData is in the process of completing its acquisition of CNT, which may occur by the time this article goes to press. CNT spokespeople weren't available for comment due to regulatory restrictions.)

Brocade. Brocade's roots are planted in the core/edge switch market it has dominated for a number of years. Its original director, the SilkWorm 12000, in many respects didn't meet the director definition (e.g., no hot code load initially and 64-port domains linked with ISLs). But the SilkWorm 24000 does, with 128 ports and non-blocking, full duplex throughput; it also uses a mesh architecture internally. Brocade boasts a unified product family from the eight-port SilkWorm 3250 edge switch up to the 128-port SilkWorm 24000.

Cisco. Similar to Brocade, Cisco groups all of its products as a unit, called the MDS 9000. The high-end director, the MDS 9509 Multilayer Director, has 1.44Tb/sec non-blocking mesh throughput. Up to 224 ports can be housed in a single chassis, with up to 672 ports in a single rack. Cisco claims its chassis are designed to last 10 years, with upgrades facilitated by swapping out modules. Modules are the essence of the Cisco architecture, and may include FC, iSCSI, Fibre Channel over IP (FCIP) and multiprotocol routers.

CNT. CNT's top product is the UltraNet Multiservice Director (UMD), which features 5Tb/sec non-blocking throughput with 512 ports at 4Gb/sec per port. CNT refers to its architecture as a low-latency, single-stage core. Recently, however, McData, which is in the process of acquiring CNT, announced that to eliminate redundancy across the McData and CNT product lines, the UMD and FC/9000 directors will be discontinued. In addition to the UMD and FC/9000, CNT also offers the FC/FICON director with 256 1Gb/sec or 2Gb/sec ports, as well as the CD/9000, an ESCON channel director.

McData. McData markets its products as elements in a three-tier architecture. Its Tier 1 director, or backbone, is the Intrepid 10000, with 1Tb/sec of active non-blocking throughput for up to 256 ports and scaling to 1,024 ports. The Tier 2 Intrepid 6000, packs 140 ports in a single frame and 420 ports in a single rack. McData's directors are based on a crossbar architecture, which McData describes as providing direct, port-to-port connectivity for guaranteed throughput. Tier 3 includes McData's Eclipse and Sphereon edge switches.

Spotlight on director-class switches

This was first published in June 2005

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