User`s guide

6 Implementing an Open IBM SAN
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.
1.2.5 Attaching to a FICON channel or a FICON channel path group
When you attach multiple controllers to a channel, you can use a switch
(director) for each controller, or an ESCON or FICON channel that has a direct
connection to the channel. I/O does not flow through all the other controllers
before you get to the target controller. I/O 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 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.
FICON does not support a private connection. FICON performs frame (or packet)
multiplexing. A configuration with the serially connected controllers
communicates with the controllers simultaneously. It can multiplex I/O operations
across all controllers simultaneously. No interface cycles are wasted because of
the private connection. You can serially connect controllers with FICON.
The next question though is whether or not it is OK to serially connect DASD
control units with tape controllers. Tape generally performs much larger I/O
operations at any instant in time. Therefore, even with FICON, when you have
Tape I/O running, you can temporarily lockout some DASD I/O. Hence, it is still
better not to put tape and DASD on the same FICON channel.
Note: x must be a minimum of two and cannot exceed a maximum of eight.