IBM System storage DS6000 Series, Attachment Manual

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If the ESCON channel use is low, you can map 6 or 8 ESCON links to a single
FICON link. If the ESCON channel use is high, you can map 2 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 a storage unit. For ESCON, you can configure at least four paths in
the path group to maximize performance.
Most ESCON controllers initiate channel command execution that partially
synchronizes the lower disk drive interface with the upper channel interface. This
channel command allows you only a very short time to reconnect. The
consequence of limited time is a reconnection that can fail. Having eight paths in
the path group, 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 disk drive
interface and the upper channel interface. The number of paths in the path group
depends 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 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.
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 a private
connection. You can serially connect controllers with FICON without performance
degradation.
You must also consider the implications of serially connecting disk drive control
units with tape controllers. Tape generally performs much larger I/O operations at
any instant in time. Therefore, even with FICON, when you run tape I/O, you can
temporarily lockout some disk drive I/O operations. It is still better to connect tape
and disk drive to different FICON channels.
Chapter 10. Attaching to an IBM S/390 or IBM eServer zSeries host
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