Specifications

supports 256 logical paths per FICON link (compared to just 64 for ESCON), 128
logical paths per logical subsystem, and 2048 logical paths for each ESS.
Before FICON, you could only connect with a fibre-channel and use the
fibre-channel protocol with feature code 3022. Feature code 3023, with a 50-micron
single-mode fiber cable increases the point-to-point distance from 500 m (1500 ft) to
10 km (6.2 mi). The increased distance provides greater configuration options with
zSeries host systems with FICON host adapters.
Attachment considerations
This section describes some things you should consider before you configure your
system with a FICON interface.
Setting up ESCON and FICON links
If the system requires x ESCON links, where x is the number of links to get the
performance and availability attributes you want, you must consider the number of
FICON links you need. For example, you can map 4 ESCON links to a single
FICON link and maintain approximately equivalent performance. If the ESCON
channel use is low, you can map 6 or 8 ESCON links to a single FICON link.
Multipathing for ESCON and FICON
Consider the difference between the path groups when you compare FICON to
ESCON. For example, for ESCON, you can configure 4 or 8 paths per path group
from a host to an ESS. For ESCON, you want at least four paths in the path group
to maximize performance. Most ESCON controllers initiate channel command
execution that partially synchronizes the lower DASD interface with the upper
channel interface. This channel command only allows you a very short time to
reconnect. The consequence is a reconnection that can fail. When you have eight
paths in the path group, it minimizes the number of missed reconnections.
Increasing the number of path groups does not minimize the number of missed
reconnections substantially. If you use eight paths in path groups, you can increase
the overall throughput.
For FICON controllers, there is no synchronization between the lower DASD
interface and the upper channel interface. The number of paths in the path group
depend on the throughput requirement. If it takes x paths to satisfy the throughput
requirement, where x is the number of paths, set the path group to x.
Note: x must be a minimum of two and cannot exceed a maximum of eight.
Attaching to a FICON channel or a FICON channel-path group
When you attach multiple controllers to a channel, you are connecting serially. You
can use a switch (director) for each controller or an ESCON or FICON channel that
has a direct connection to the controller. I/O activity does not flow through all the
other controllers before you get to the target controller. I/O activity goes directly to
the target controller. When multiple controllers are connected to a channel through
a switch, you create the logical equivalent of the parallel interconnection.
With the parallel interface and with the ESCON interface, the channel and controller
communicate to form a private connection. None of the other controllers on the
channel can communicate with the channel while this private connection is in place.
The private connection supports input and output activity between the channel and
the controller. It can run slowly, depending upon the factors that affect the controller
and the device. The protocol does not allow any of the serially connected
controllers to use any spare cycles. The result is poor performance.
Chapter 7. Attaching to an IBM S/390 or IBM eServer zSeries host 79