The term "fabric" is thrown around a lot in product marketing. The term sounds cool and is sufficiently vague to cover a multitude of meanings. I have been asking vendors what they think a fabric is. Somekh defined fabric as the hardware--switches and services like QoS that the network provides in delivering frames from node to node. It's the fabric, he says, that Voltaire has 10+ years experience in delivering with Infiniband. Data center Bridging Ethernet has the same set of requirements as Infiniband: lossless, reliable, redundant and fast connectivity in a different Layer 2 protocol.
But those requirements include new problems. With Ethernet, when a network port gets congested, the accepted practice is to drop frames and let the upper-layer protocols perform the recovery. It works well enough. But in lossless Ethernet, frames can't be dropped, which can actually lead to a worsening condition. Priority Pause is developed so that the congested receiver can tell the adjacent Layer 2 device to stop sending frames until the congestion is relieved, which should provide temporary relief. Of course, pausing transmission leads to increased latency between nodes. Which is worse, dropped frames or delayed transmission? I suppose the action that takes longer to recover from.
Priority pause, however, has intelligence beyond simply telling an adjacent Layer 2 device to stop sending. What we want is to pause low-priority traffic and favor high-priority traffic, and that implies tagging traffic with priority marks. In addition, traffic can be aggregated from many switch ports onto a single output port, and when it receives a priority pause, it has to ferret out the origin of the flow originated and make a determination about how to proceed. Should it start to queue traffic or should it signal a priority pause to its adjacent neighbor? All of this happens under the covers, but how it happens, the decision-making process, will impact how effectively priority pause works.
Voltaire recently announced its Unified Fabric Manager for 10Gb Ethernet which, in part, provides intelligent path-management so that congestion can be handled before it becomes a problem. Equally important, UFM can also detect potential problems and provide advice like an expert system to IT on how to better connect nodes to maximize thoughput and minimize delay. UFM can do it across vendor product lines that include Blade Networks and HP switches. It's an interesting message that I have yet to hear from other switch vendors. I don't claim to know if UFM works as the vendor claims, or if it can scale. I don't know enough about the product to make that assessment, but the kinds of control and analysis they are claiming is compelling. Voltaire has a demo at Interop that I will check out.