IBM Z9, Planning Manual

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Managing Logical Paths for ESCON and FICON Channels
This section describes logical paths, explains overall system considerations, and
makes specific configuration recommendations.
Definition
A logical path is a logical connection between a control unit and an ESCON channel
(TYPE=CNC or TYPE=CTC), a FICON channel (TYPE=FCV) attached to an
ESCON Director, or a FICON channel (TYPE=FC). (FICON Express2 and FICON
Express4 features do not support TYPE=FCV channels.) Logical paths are
important because each sharing LP on a CPC requires that a logical path be
established between an ESCON or FICON channel and a control unit for I/O
operations to occur.
Logical paths do not exist for ESCON channels attached to a 9034 ESCON
Converter Model 1 (TYPE=CBY or TYPE=CVC).
Logical paths do not exist for coupling facility channel paths (TYPE=CFP),
integrated cluster bus channel paths (TYPE=CBP), InfiniBand internal coupling
channel paths (TYPE=ICP), Open Systems Adapter channel paths (TYPE=OSC,
TYPE=OSD or TYPE=OSE, TYPE=OSN), internal queued direct communication
(HiperSockets) channel paths (TYPE=IQD), or fibre channel protocol channel paths
(TYPE=FCP).
Control Unit Allocation of Logical Paths
Control units allocate logical paths to channels dynamically on a
first-come-first-served basis. Control units do not manage the allocation of logical
paths but instead allow channels to compete for logical paths until all of the control
unit’s logical paths are used.
Why Manage Logical Paths?
The ESCON and FICON environments (the use of ESCON channels, FICON
Express channels, FICON Express2 and FICON Express4 channels, and ESCON
and FICON Directors) greatly enhances the connectivity potential for control units.
In addition, you can define shared channels that can request additional logical
paths. However, control units can only allocate a limited number of logical paths
(see Table 2-3 on page 2-8) in relation to the number of logical paths that channels
can request. In configurations where channels request more logical paths than a
control unit can allocate, you must manage logical paths to help ensure that the I/O
operations you intend take place.
The FICON Express2 and FICON Express4 SX and LX features support all of the
functions of FICON Express, with the exception of support for channel path type
FCV. FICON Express2 and FICON Express4, however, offers increased connectivity
in the same amount of physical space, and offers the possibility of increased
performance. Up to 240 FICON Express2 and FICON Express4 channels on a z9
EC (and 112 channels on a z9 BC) can be employed to greatly expand connectivity
and throughput capability. The FICON connectivity solution is based on
industry-standard Fibre Channel technology and leverages our exclusive native
FICON architecture. For detailed information, see Input/Output Configuration
Program User’s Guide , SB10-7037.
Chapter 2. Planning Considerations
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