IBM Z9, Planning Manual

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In situations where ESCON CTC communication is required among LPs that exist
on two or more CPCs, shared channels can reduce even further channel and other
hardware requirements and their associated cost.
ESCON CTC configurations are well-suited to take advantage of the consolidation
benefits associated with shared channels. CTC/CNC and CTC/FCV pairs used for
ESCON CTC communications have no limitation on the number of logical paths that
can be established between them. The only limitations are the number of control
units that can be defined for an ESCON CTC channel and the performance
expectations you determine for your configuration.
Infrequently Used ESCON or FICON Control Units: ESCON or FICON control units
not frequently used can make use of shared channels. You can attach such a
control unit to a shared channel that is also attached to other, more frequently used
control units without adding greatly to the channel utilization of the shared channel.
The control unit within the Director is a good example of this.
Notes:
1. You cannot define a control unit (or multiple control units with common I/O
devices) to a mixture of shared and unshared channel paths in the same
IOCDS.
2. You cannot define more than one control unit with the same CUADD to the
same link on a Director (or point-to-point) if the attaching CHPIDs are shared.
Unshared ESCON or FICON Channel Recommendations
Not all ESCON or FICON configurations benefit from the use of shared channels.
There are some configurations where use of an unshared channel is more
appropriate. Consider the following:
v
Logical Path Limitations of the Control Unit
While many ESCON control units can communicate with multiple LPs at a time
using multiple logical paths, there are some ESCON-capable control units that
can only communicate with one LP at a time. For example, consider the 3174
Establishment Controller (Models 12L and 22L). When configured in non-SNA
mode, the 3174 establishes only one logical path at a time. A shared channel
would offer no connectivity benefit in this situation. However, if you defined an
unshared, reconfigurable channel to the 3174, it would allow you to dynamically
reconfigure the channel for any LP that had to communicate with the 3174 at a
given time.
v Channel Utilization
Typically, the channel utilization of shared channels will be greater than unshared
channels.
If you use shared channels to consolidate channel resources, you must consider
the channel utilization of all the channels you consolidate. The channel utilization
of a shared channel will roughly equal the sum of the channel utilizations of each
unshared channel that it consolidates. If this total channel utilization is capable of
decreasing performance, you should consider using unshared channels or a
different configuration of shared and unshared channels to meet your connectivity
needs.
Dynamically Managed CHPIDs
A key aspect of the Intelligent Resource Director (IRD) provided by z/OS’s WLM
component is Dynamic CHPID Management (DCM). DCM provides the ability to
have the system automatically move the available channel bandwidth to where it is
Chapter 2. Planning Considerations
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