Storage & Data Management: Two-Hop FICON Cascading

by Steve Guendert
May 5, 2009

Here I will address the answer to a question I’m frequently asked: Whether two-hop cascaded FICON will be supported in the future.

Prior to the introduction of support for cascaded FICON director connectivity on IBM zSeries mainframes in January 2003, only a single level of FICON directors was supported for connectivity between a processor and peripheral devices. Cascaded FICON allows a FICON Native (FC) channel or a FICON Channel-to-Channel (CTC) to connect a zSeries/ System z server to another similar server or peripheral device such as a disk, tape library, or printer via two FICON directors or switches connected by an Inter-Switch Link (ISL). IBM, storage OEMs, and FICON switch/director vendors support the flow of traffic from the processor through two FICON directors connected via an ISL to the peripheral devices, such as disk and tape.

A FICON channel in FICON Native mode connects one or more processor images to an FC link, which connects to the first FICON director, then dynamically through the first director to one or more ports, and from there to a second cascaded FICON director. From the second director there are Fibre Channel links to FICON Control Unit Ports (CUPs) on attached devices. These FICON directors typically are geographically separate, providing greater flexibility and fiber cost savings for a business continuity architecture.

Initial support by IBM is limited to a single hop between cascaded FICON directors; however, the directors can be configured in a hub-star architecture with up to 24 directors in the fabric.

Smaller port count FICON switches with FICON CUPs capabilities have been available now for nearly six years (the first of these was the McDATA 3232). These FICON switches with CUP support go through the same rigorous qualification process at IBM Poughkeepsie as FICON directors. Due to the mechanical nature of a tape drive (i.e., moving parts), the very best tape drives in the world are a three 9’s device in terms of reliability and availability. While the argument can be made that DASD also has moving mechanical parts, DASD is more reliable than tape drives. (If it weren’t, production data would be stored on tape drives.) FICON directors are five 9’s devices. FICON switches don’t typically have all the redundant features as do the large port count FICON directors. Therefore, FICON switches typically aren’t a five 9’s device, and depending on switch model and configuration, typically are three or four 9’s devices.

When would two-hop cascaded FICON be valuable? When I’m asked if two-hop cascaded FICON will be supported in the future, the question is typically asked by an end user who is looking at how to best connect tape drives into a cascaded FICON environment. For example, let’s assume we have a cascaded FICON environment that has the FICON ISLs between FICON directors located at two sites. The end user in this example would like to do electronic tape vaulting at the remote site. If the end user is using tape drives that are direct fibre connect (no intervening separate control unit), their only current supported choice is to plug each drive into the FICON director at the remote site. This not only uses many director ports, but it can be quite costly.