Synchronous SAN Sets Fibre Channel Distance Record

Fibre Channel distance barriers were shattered in a recent demonstration led by Vtesse Networks and Hitachi Data Systems.

The two companies demonstrated a representative synchronous SAN application over a range of link lengths up to 600 kilometers, "six times farther than previously thought possible for a data application to be synchronously transmitted over a Fibre Channel link," the companies announced.

"It was done for a specific customer requirement at around 400 kilometers, but we pushed it just to see what would happen," said Aidan Paul, CEO of Vtesse, a UK Gigabit optical networking firm. The unnamed customer is a financial institution that has trading systems in London and its main IT infrastructure outside London.

Asked what this means for Fibre Channel distance limits, Paul replied, "There is a practical limit, but not a hard one. It would be possible to go to 800 kilometers synchronous, but likely as not, most organizations would start to regionalize the IT infrastructure after that. There would also be a further gradual drop in throughput. I can't really see someone wanting to do synchronous across the Atlantic, for example, but asynchronous would be fine."

Vtesse and HDS worked with Transmode Systems AB and Nishan Systems on the synchronous SAN. The HDS TrueCopy 9900 disk storage system was used to synchronously update an Oracle database over a Vtesse Networks Gigabit Ethernet transmission system, using Transmode CWDM equipment and single-mode fiber supplied by Corning. A Nishan Systems IPS 3300 multi-protocol switch was used to convert native Fibre Channel from the HDS equipment into Gigabit Ethernet and thereby remove the usual link length restrictions of the Fibre Channel format.

Conventional Fibre Channel transmission "poses serious restrictions on realizable link lengths for synchronous SAN applications such as data replication using disk mirroring," according to the companies. Throughput capacity of a Fibre Channel system falls very quickly over link lengths greater than 100 kilometers due to the inherent time delay, or latency, of the fiber link length and insufficient buffering in Fibre Channel fabric switches.

This reduction in throughput capacity slows the operation of SAN applications. The trial demonstrates that successful operation involving acceptable performance degradation can be achieved over distances of up to 600 kilometers, the firms contend.

"This trial not only shows that the inherent length restrictions of Fibre Channel can be overcome, but it also demonstrates that synchronous data replication can now be realized between data centers anywhere within the UK," Paul asserts. "Another benefit is that more cost-effective and widely available Gigabit Ethernet networks can be used to interconnect SAN applications. This arrangement reduces costs as well as increases the range of options open to companies requiring data replication and protection over national rather than regional distances."

Delving into the Demonstration Details

The trial arrangement used the Nishan IPS 3300 IP storage switches to convert the Fibre Channel format from the HDS storage system into Gigabit Ethernet and then transmit this format natively over the Transmode CWDM system. The Nishan equipment used frame buffering and data processing techniques to reduce the length restrictions caused by the frame-by-frame acknowledgement methods used in Fibre Channel, thereby extending the length capability to several thousand kilometers without losing the advantages of true synchronous operation.

The Transmode equipment used for the trial provides a multi-channel GbE transmission system that can be extended almost indefinitely over national distances. Such link lengths usually require high-cost DWDM equipment.

The Oracle database application involved simple row insertions and was chosen specifically by HDS to simulate a real-life data replication requirement of a typical major UK financial institution. Timings for a representative Oracle database operation involving 82.5GB of data were measured both for local operation with zero link length and over a range of link distances between 50 to 600 kilometers. Data throughput in MB/second was also measured.

Throughput capacity of the initial data copy was 42MB/second in local mode and within 83% of that at the maximum link length of 600 kilometers, with timings for the operation of the data replication application increasing by less than 33% over this distance, the companies said.

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