IBM z13s, Technical Manual

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Chapter 5. Central processor complex channel subsystem
189
purchased memory. This fixed size memory for HSA eliminates the requirement of additional
planning for initial I/O configuration and pre-planning for future I/O expansions.
The objects are always reserved in the z13s HSA during POR, no matter if they are defined in
the IOCDS for use or not:
򐂰 Three CSSs
򐂰 Fifteen LPARs in each CSS0 to CSS1
򐂰 Ten LPARs in CSS2
򐂰 Subchannel set 0 with 63.75 K devices in each CSS
򐂰 Subchannel set 1 with 64 K minus one device in each CSS
򐂰 Subchannel set 2 with 64 K minus one device in each CSS
5.1.1 Multiple logical channel subsystems
In z Systems architecture, a single channel subsystem can have up to 256 channel paths
defined, which limited the total numbers of I/O connectivities on a z Systems server to within
256.
The introduction of multiple LCSSs has enabled a z Systems server to have more than one
channel subsystems (logically), while maintaining the same manner of I/O processing within
each LCSS. Also, a logical partition (LPAR) is now attached to a specific logical channel
subsystem, which makes the extension of multiple logical channel subsystems transparent to
the operating systems and applications. The multiple image facility (MIF) in the structure
enables resource sharing across logical partitions within a single LCSS or across the LCSSs.
The multiple LCSS structure has extended z Systems’ total number of I/O connections to
support a balanced configuration for the growth of processor and I/O capabilities.
A one-digit number ID starting from 0 (CSSID) is assigned to an LCSS, and a one-digit
hexadecimal ID (MIF ID) starting from 0 is assigned to an LPAR within the LCSS.
Subchannels
A
subchannel
provides the logical appearance of a device to the program and contains the
information required for sustaining a single I/O operation. Each device is accessible by using
one subchannel in a channel subsystem to which it is assigned according to the active
IOCDS of the z Systems server. A subchannel set (SS) is a collection of subchannels within a
channel subsystem, and the maximum number of subchannels in a subchannel set
determines how many devices can be accessible to a channel subsystem.
In z Systems architecture, the first subchannel set of an LCSS can have 63.75K subchannels
(with 0.25K reserved), with a subchannel set ID (SSID) as 0. By enabling the multiple
subchannel sets, which are described in 5.1.2, “Multiple subchannel sets” on page 190, extra
subchannel sets are available to increase the device addressability of a channel subsystem.
Channel paths
A
channel path
provides a connection between the channel subsystem and control unites,
that the channel subsystem uses to communicate with I/O devices. Depends on the type of
CPC drawer repair: For z13s servers, the CPC drawer repair actions are always disruptive.
Note: The word “channel subsystem” has same meaning as “logical channel subsystem”
in this chapter unless otherwise indicated.