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Mainframe Solutions

FICON MultiHop arrives

by Dr.Steve.Guendert on ‎07-12-2017 11:08 AM (5,787 Views)

It is an exciting time to be involved in mainframe I/O!  New processors with significant technology improvements are on the horizon which also likely will provide significant  I/O performance gains, especially to users implementing the latest flash storage technology.  One of the latest features to be announced in the FICON universe, that may slip under your radar, is FICON Multihop.  FICON Multihop is now officially supported by IBM Z. 


Recall that IBM originally announced FICON channels for the S/390 9672 G5 processor in May of 1998. Over the ensuing 19 years, FICON has rapidly evolved from 1 Gbit FICON Bridge Mode (FCV) to the current 16 Gbit FICON Express channels. The nine generations of FICON channels have each offered increased performance and capacity.  The most important enhancements relate to the fibre channel upper level protocol (ULP) employed by FICON. The first FICON ULP was FC-SB-2, which supported native FICON storage devices, channel-to-channel connections, FICON directors (switches), as well as a number of other key features. However, the FC-SB2 ULP did not support switched paths over multiple directors. When IBM introduced the FC-SB-3 ULP in January of 2003, this limitation was addressed. While FC-SB-2 employed a single byte link address that specified just the switch port on the director, FC-SB-3 employs a 2-byte link address where the first byte is the address of the destination director and the second byte is the address of the port on the destination director.  This is more commonly known as cascaded FICON. 


One of the things about cascaded FICON that has been an antithesis to the strict, programmed data traffic control of mainframe environments has been the handling of data traffic on the interswitch links (ISLs).  Since 2003, there have been several changes with how the FICON switching devices send fibre channel frame traffic between each other, and how that traffic has been managed.  One constant since 2003 has been what is known as the “one-hop rule”:  From a channel to a control unit/storage device, data traffic may only traverse two FICON switching devices connected by ISLs.  The ISLs between the two switching devices are known as the “hop”.  To allow for connecting Fibre Channel over IP (FCIP) extension switches, and using special connections between FICON switching devices that attach the switch backplanes together (in lieu of using a standard port and ISL), IBM and the FICON SAN vendors supported what are known as “hops of no concern”. 


Many IBM Z installations have implemented business continuity architectures that have more than two geographically dispersed sites.  These three (or more) site architectures were a natural evolution to deal with the increased threat from regional disasters such as Hurricanes Katrina, Sandy, or acts of terrorism.  In these architectures, synchronous DASD replication is performed between two sites located in close proximity for replication performance reasons, and additional sites are located out of region, at longer distances from the primary site. Asynchronous DASD replication would be used to replicate data to these sites.  One of the challenges created with these three or more site architectures was how to remain in a one-hop FICON configuration (for support reasons) in the event of a site failure and failover.


To address this challenge, IBM tested, and now supports what is called FICON Multihop.  IBM announced this support with the June 2017 publication of an IBM White Paper FICON Multihop Requirements and Configurations (IBM Document WP102704) written by P.J. Catalano (IBM) and myself (Brocade).  Through this whitepaper and supporting IBM Z Storage Area Network (SAN) qualification letters, IBM announced support for a new set of cascaded FICON configurations for FICON interswitch links (ISLs) called FICON Multihop. FICON Multihop allows support for cascading up to four switches, three hops, expanding the previous restriction of two cascaded switches, one hop. Multihop is only supported using traditional static routing methods, and is not supported in conjunction with FICON Dynamic Routing (FIDR).  IBM Z servers using Multihop have advantages for simplifying configurations in a single site or across multiple sites with ISL and cascaded FICON directors:


  • Support multiple configurations for easy deployment of GDPS
  • Improve utilization of FICON directors and switches
  • Simpler and easier to manage configurations for SAN availability

The focus of the white paper is to illustrate the new configurations that are supported in a Multihop environment. For more details, please read the paper, and/or attend my session at SHARE Providence.


In summary, a significant technological innovation has occurred over the past fourteen years since cascaded FICON debuted in 2003.  These technical innovations have driven the need for further innovations in how the interswitch links manage traffic flows, and how the end user manages the ISLs.  FICON Multihop is the latest such management innovation. There are some design implications that need to be considered with the use of FICON Multihop environments, FICON Multihop provides significant configuration flexibility and management improvements over the older FICON single hop policy.


Dr. Steve