A single protocol in your SAN?

Typical use of the fibre channel is inefficient, and can be made better

A newly proposed standard, dubbed FCoE (Fibre Channel over Ethernet), aims to bring together the efficiency of the FC (Fibre Channel) transport and the ubiquity of Ethernet.

It's no mystery that the majority of storage networks runs on fibre, and FC is the transport protocol that rules data exchanges over that medium. However, the FC fabric is pretty much limited to the storage back end, and corporate data has to jump over the ubiquitous Ethernet to reach local and remote users or to move to a remote SAN, for example.

A typical use of FCoIP (Fibre Channel over IP) is to link remote fabrics, but it can also connect neighbouring SAN islands when merging them is not a viable option. The FCoIP protocol essentially bags each FC frame in a TCP/IP container . At the end of the IP segment, that bag is removed and the original frame is delivered to the FC fabric at the remote site.

Sound inefficient? You bet it is. Can we do better? After listening to Claudio DeSanti, vice chairman of the T11 Committee and technical leader of Cisco's Data Center Business Unit, I think FCoE can.

What FCoE proposes is essentially running FC directly over Ethernet, without using another transport layer (TCP/IP serves as this transport layer for FCoIP). With FCoE, the FC frame is wrapped inside an Ethernet frame, all with minimal overhead as compared to FCoIP.

One of the problems that the new standard has to solve is that it's perfectly acceptable to discard a frame in the Ethernet world, while similar behavior is a mortal sin in FC networks. For example, Ethernet drops frames when the amount of incoming traffic exceeds the capacity of the buffers in a switch. Dropping a frame doesn't cause data loss, because TCP will retransmit frames for which there is no acknowledgement, but that retransmission causes a delay.

By contrast, the obsessively efficient FC protocol never drops a frame. Each end of a FC connection keeps the other party informed of how many buffers are available, and frames are sent only if there is enough buffer capacity at the destination port.

FCoE aims to mimic that behavior via PAUSE frames , a little-known and rarely used extension to the Ethernet protocol that, as its name implies, suspends the flow when there is a congestion point.

It may take years before FCoE products reach our SANs, but I'll speculate that the new standard will make it easier and more affordable to connect your servers to the fabric without doubling with another transport protocol such as iSCSI. You may still need FCoIP to bridge remote fabrics, but local connections can speak FC regardless of the medium.

Is the future of iSCSI threatened by FCoE? It's hard to tell at this point, but I think the usual factors such as cost and friendliness of the solutions will determine the outcome.

Perhaps the most interesting aspect of FCoE is that it will make it possible to consolidate FC and Ethernet into a single, reliable, fast fabric -- 10 Gig Ethernet, anyone?

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Mario Apicella

InfoWorld

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