The I/O subsystem uses control
unit headers to represent a set of physical control units that physically
or logically attach I/O devices in common. For example, a logical
channel subsystem can use the control unit header to establish a queue
for I/O requests for the devices associated with the attaching physical
control units.
Note: Control unit headers were previously called
logical control units.
IOCP builds control unit headers from the information specified
in the CNTLUNIT and IODEVICE statements. If a device attaches to more
than one physical control unit, the physical control units form part
of (or the whole of) a control unit header.
IOCP builds a control unit header for the following control units:
- Each physical control unit with no I/O devices attached.
- Each physical control unit that does not share any I/O devices
with other physical control units.
- Each group of two or more control units that share devices between
them.
Figure 1 shows
examples of valid control unit headers.
When a physical control unit or a group of control units that share
devices have channel paths in multiple logical channel subsystems
(CSSs), IOCP builds a control unit header (CUH) for each CSS. The
CUH in each CSS contains only the channel paths from the physical
control units associated with that CSS. The CUHs in each CSS are numbered
independently of each other. The devices assigned to the physical
control unit or group of control units become subchannels in each
CSS and are numbered independently of each other. The subchannels
in each CSS are associated with the CUH in the same CSS and its channel
paths. In Figure 1,
CUH 3 is in CSS 0 with paths 01 and 02. CUH 19 is in CSS 1 with paths
21 and 24. A subchannel is generated in each CSS for device C. The
subchannel in CSS 0 is associated with CUH 3 and the subchannel in
CSS 1 is associated with CUH 19.
As
it builds each control unit header, IOCP assigns a unique hexadecimal
identifier to the control unit header in each CSS. It also ensures
that the control unit header is valid and that it complies with all
the following requirements:
- A maximum of eight physical control units are in one control unit
header (CUH).
- A maximum of eight channel paths can attach to one CUH for each
logical channel subsystem (CSS) in a CPC.
- Each occurrence of a channel path counts as one channel path.
- Only one control unit can belong to a CUH if the control unit is an ESCON CTC or FICON® CTC control unit
or if the control unit attaches to any of the following types of channel path:
- CBP
- CBS
- CFP
- CFS
- CIB
- CL5
- CL6
- CS5
- FCP
- ICP
- IQD
- OSC
- OSD
- OSE
- OSH
- OSM
- OSN
- OSX
- Only one channel path per CSS can belong to a CUH if the control unit is an ESCON CTC or FICON CTC or the control
unit attaches to any of the following types of channel path:
- FCP
- IQD
- OSC
- OSD
- OSE
- OSH
- OSM
- OSN
- OSX
If the control unit has channel paths in multiple CSSs, a CUH is built for each CSS with one
path each.
- A physical control unit can belong to only one CUH in a CSS.
- Parallel physical control units within a CUH must all have the
same characteristics (SHARED=Y or N; PROTOCL=D, S, or S4).
- All channel path types (TYPE keyword) attaching to a CUH must meet the following restrictions:
- All CNC, FCV, or FC
- All CFS or CBS
- All CFP, CBP, ICP, CIB, CL5, CL6,
or CS5
- All CBY or CVC
- All shared or all unshared (the SHARED, NOTPART, or PART or PARTITION keywords must be
consistently specified for all channel paths).
- A maximum number of control unit headers cannot be exceeded for
each CSS, depending on the CPC (see Machine limits and rules).
Figure 2 shows
an example of a control unit header that is incorrect. Control unit
header 5 is incorrect because the physical control units have a combined
total of more than eight channel paths in a CSS.
