Central processor complex channel subsystem
This chapter describes the concepts of the z15 channel subsystem, including multiple channel subsystems and multiple subchannel sets. It also describes the technology, terminology, and implementation aspects of the channel subsystem.
This chapter includes the following topics:
5.1 Channel subsystem
Channel subsystem (CSS) is a collective name of facilities that Z servers use to control I/O operations.
The channel subsystem directs the flow of information between I/O devices and main storage. It allows data processing to proceeded concurrently with I/O processing, which relieves data processors (central processor (CP) and Integrated Facility for Linux [IFL]) of the task of communicating directly with I/O devices.
The channel subsystem includes subchannels, I/O devices that are attached through control units, and channel paths between the subsystem and control unites. For more information about the channel subsystem, see 5.1.1, “Multiple logical channel subsystems”.
The design of IBM Z servers offers considerable processing power, memory size, and I/O connectivity. In support of the larger I/O capability, the CSS structure is scaled up by introducing the multiple logical channel subsystem (LCSS) since z990, and multiple subchannel sets (MSS) since z9.
An overview of the channel subsystem for z15 servers is shown in Figure 5-1. z15 servers are designed to support up to six logical channel subsystems, each with four subchannel sets and up to 256 channels.
Figure 5-1 Multiple channel subsystems and multiple subchannel sets
All channel subsystems are defined within a single configuration, which is called I/O configuration data set (IOCDS). The IOCDS is loaded into the hardware system area (HSA) during a central processor complex (CPC) power-on reset (POR) to start all of the channel subsystems.
On z15 servers, the HSA is pre-allocated in memory with a fixed size of 256 GB, which is in addition to the customer purchased memory. This fixed size memory for HSA eliminates the requirement for more planning of the initial I/O configuration and pre-planning for future I/O expansions.
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CPC drawer repair: The HSA can be moved from one CPC drawer to a different drawer in an enhanced availability configuration as part of a concurrent CPC drawer repair (CDR) action.
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The following objects are always reserved in the z15 HSA during POR, whether they are defined in the IOCDS for use:
•Six CSSs
•A total of 15 LPARs in each CSS0 to CSS4
•A total of 10 LPARs in CSS5
•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
•Subchannel set 3 with 64 K minus one device in each CSS
5.1.1 Multiple logical channel subsystems
In the z/Architecture, a single channel subsystem can have up to 256 channel paths that are defined, which limited the total numbers of I/O connectivity on older Z servers to 256.
The introduction of multiple LCSSs enabled an IBM Z server to have more than one channel subsystems logically, while each logical channel subsystem maintains the same manner of I/O processing. Also, a logical partition (LPAR) is now attached to a specific logical channel subsystem, which makes the extension of multiple logical channel subsystems not apparent to the operating systems and applications. The multiple image facility (MIF) in the structure enables resource sharing across LPARs within a single LCSS or across the LCSSs.
The multiple LCSS structure extended the Z servers’ total number of I/O connectivity 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.
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Note: The phrase channel subsystem has same meaning as logical channel subsystem in this section, unless otherwise stated.
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Subchannels
A subchannel provides the logical appearance of a device to the program and contains the information that is 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 server.
A subchannel set (SS) is a collection of subchannels within a channel subsystem. The maximum number of subchannels of a subchannel set determines how many devices are accessible to a channel subsystem.
In z/Architecture, the first subchannel set of an LCSS can have 63.75 K subchannels (with 0.25 K reserved), with a subchannel set ID (SSID) of 0. By enabling the multiple subchannel sets, extra subchannel sets are available to increase the device addressability of a channel subsystem. For more information about multiple subchannel sets, see 5.1.2, “Multiple subchannel sets” on page 208.
Channel paths
A channel path provides a connection between the channel subsystem and control units that allows the channel subsystem to communicate with I/O devices. Depending on the type of connections, a channel path might be a physical connection to a control unit with I/O devices, such as FICON, or an internal logical control unit, such as HiperSockets.
Each channel path in a channel subsystem features a unique 2-digit hexadecimal identifier that is known as a channel-path identifier (CHPID), which ranges 00 - FF. Therefore, a total of 256 CHPIDs are supported by a CSS, and a maximum of 1536 CHPIDs are available on a z15 server with six logical channel subsystems.
By assigning a CHPID to a physical port of an I/O feature adapter, such as FICON Express16SA, or a fanout adapter (ICA SR) port, the channel subsystem connects to the I/O devices through these physical ports.
A port on an I/O feature card features a unique physical channel identifier (PCHID) according to the physical location of this I/O feature adapter, and the sequence of this port on the adapter.
In addition, a port on a fanout adapter has a unique adapter identifier (AID), according to the physical location of this fanout adapter, and the sequence of this port on the adapter.
A CHPID is assigned to a physical port by defining the corresponding PCHID or AID in the I/O configuration definitions.
Control units
A control unit provides the logical capabilities that are necessary to operate and control an I/O device. It adapts the characteristics of each device so that it can respond to the standard form of control that is provided by the CSS.
A control unit can be housed separately or can be physically and logically integrated with the I/O device, channel subsystem, or within the Z server.
I/O devices
An I/O device provides external storage, a means of communication between data-processing systems, or a means of communication between a system and its environment. In the simplest case, an I/O device is attached to one control unit and is accessible through one or more channel paths that are connected to the control unit.
5.1.2 Multiple subchannel sets
A subchannel set is a collection of subchannels within a channel subsystem. The maximum number of subchannels of a subchannel set determines how many I/O devices that a channel subsystem can access. This number also determines the number of addressable devices to the program (for example, an operating system) that is running in the LPAR.
Each subchannel has a unique four-digit hexadecimal number 0x0000 - 0xFFFF. Therefore, a single subchannel set can address and access up to 64 K I/O devices.
As with the z13 server, the z15 support four subchannel sets for each logical channel subsystem. It can access a maximum of 255.74 K devices for a logical channel subsystem and a logical partition and the programs that are running on it.
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Note: Do not confuse the multiple subchannel sets function with multiple channel subsystems.
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Subchannel number
The subchannel number is a four-digit hexadecimal number 0x0000 - 0xFFFF, which is assigned to a subchannel within a subchannel set of a channel subsystem. Subchannels in each subchannel set are always assigned subchannel numbers within a single range of contiguous numbers.
The lowest-numbered subchannel is subchannel 0, and the highest-numbered subchannel includes a subchannel number equal to one less than the maximum numbers of subchannels that are supported by the subchannel set. Therefore, a subchannel number is always unique within a subchannel set of a channel subsystem and depends on the sequence of assigning.
With the subchannel numbers, a program that is running on an LPAR (for example, an operating system) can specify all I/O functions relative to a specific I/O device by designating a subchannel that is assigned to the I/O devices.
Normally, subchannel numbers are used only in communication between the programs and the channel subsystem.
Subchannel set identifier
While introducing the MSS, the channel subsystem is extended to assign a value 0 - 3 for each subchannel set, which is the SSID. A subchannel can be identified by its SSID and subchannel number.
Device number
A device number is an arbitrary number 0x0000 - 0xFFFF, which is defined by a system programmer in an I/O configuration for naming an I/O device. The device number must be unique within a subchannel set of a channel subsystem. It is assigned to the corresponding subchannel by channel subsystem when an I/O configuration is activated. Therefore, a subchannel in a subchannel set of a channel subsystem includes a device number together with a subchannel number for designating an I/O operation.
The device number provide a means to identify a device, independent of any limitations that are imposed by the system model, configuration, or channel-path protocols.
A device number also can be used to designate an I/O function to a specific I/O device. Because it is an arbitrary number, it can easily be fit into any configuration management and operating management scenarios. For example, a system administrator can set all of the z/OS systems in an environment to device number 1000 for their system RES volumes.
With multiple subchannel sets, a subchannel is assigned to a specific I/O device by the channel subsystem with an automatically assigned subchannel number and a device number that is defined by user. An I/O device can always be identified by an SSID with a subchannel number or a device number. For example, a device with device number AB00 of subchannel set 1 can be designated as 1AB00.
Normally, the subchannel number is used by the programs to communicate with the channel subsystem and I/O device, whereas the device number is used by a system programmer, operator, and administrator.
Device in subchannel set 0 and extra subchannel sets
An LCSS always includes the first subchannel set (SSID 0), which can have up to 63.75 K subchannels with 256 subchannels that are reserved by the channel subsystem. Users can always define their I/O devices in this subchannel set for general use.
For the extra subchannel sets enabled by the MSS facility, each has 65535 subchannels (64 K minus one) for specific types of devices. These extra subchannel sets are referred as alternative subchannel sets in z/OS. Also, a device that is defined in an alternative subchannel set is considered a special device, which normally features a special device type in the I/O configuration.
Currently, a z15 server that is running z/OS defines the following types of devices in another subchannel set, with proper APAP or PTF installed:
•Alias devices of the parallel access volumes (PAV).
•Secondary devices of GDPS Metro Mirror Copy Service (formerly Peer-to-Peer Remote Copy [PPRC]).
•FlashCopy SOURCE and TARGET devices with program temporary fix (PTF) OA46900.
•Db2 data backup volumes with PTF OA24142.
The use of another subchannel set for these special devices helps reduce the number of devices in the subchannel set 0, which increases the growth capability for accessing more devices.
Initial program load from an alternative subchannel set
z15 servers support initial program load (IPL) from alternative subchannel sets in addition to subchannel set 0. Devices that are used early during IPL processing now can be accessed by using subchannel set 1, subchannel set 2, or subchannel set3 on a z15 server. This configuration allows the users of Metro Mirror (formerly PPRC) secondary devices that are defined by using the same device number and a new device type in an alternative subchannel set to be used for IPL, an I/O definition file (IODF), and stand-alone memory dump volumes, when needed.
The display ios,config command
The z/OS display ios,config(all) command that is shown in Figure 5-2 includes information about the MSSs.
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D IOS,CONFIG(ALL)
IOS506I 11.32.19 I/O CONFIG DATA 340
ACTIVE IODF DATA SET = SYS6.IODF39
CONFIGURATION ID = L06RMVS1 EDT ID = 01
TOKEN: PROCESSOR DATE TIME DESCRIPTION
SOURCE: SCZP501 14-10-31 08:51:47 SYS6 IODF39
ACTIVE CSS: 0 SUBCHANNEL SETS CONFIGURED: 0, 1, 2, 3
CHANNEL MEASUREMENT BLOCK FACILITY IS ACTIVE
LOCAL SYSTEM NAME (LSYSTEM): SCZP501
HARDWARE SYSTEM AREA AVAILABLE FOR CONFIGURATION CHANGES
PHYSICAL CONTROL UNITS 8099
CSS 0 - LOGICAL CONTROL UNITS 3996
SS 0 SUBCHANNELS 54689
SS 1 SUBCHANNELS 58862
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
CSS 1 - LOGICAL CONTROL UNITS 4088
SS 0 SUBCHANNELS 65280
SS 1 SUBCHANNELS 65535
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
CSS 2 - LOGICAL CONTROL UNITS 4088
SS 0 SUBCHANNELS 65280
SS 1 SUBCHANNELS 65535
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
CSS 3 - LOGICAL CONTROL UNITS 4088
SS 0 SUBCHANNELS 65280
SS 1 SUBCHANNELS 65535
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
CSS 4 - LOGICAL CONTROL UNITS 4088
SS 0 SUBCHANNELS 65280
SS 1 SUBCHANNELS 65535
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
CSS 5 - LOGICAL CONTROL UNITS 4088
SS 0 SUBCHANNELS 65280
SS 1 SUBCHANNELS 65535
SS 2 SUBCHANNELS 65535
SS 3 SUBCHANNELS 65535
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Figure 5-2 Output for display ios,config(all) command with MSS
5.1.3 Channel path spanning
With the implementation of multiple LCSSs, a channel path can be available to LPARs as dedicated, shared, and spanned.
While a shared channel path can be shared by LPARs within a same LCSS, a spanned channel path can be shared by LPARs within and across LCSSs.
By assigning the same CHPID from different LCSSs to the same channel path (for example, a PCHID), the channel path can be accessed by any LPARs from these LCSSs at the same time. The CHPID is spanned across those LCSSs. The use of spanned channels paths decreases the number of channels that are needed in an installation of Z servers.
A sample of channel paths that are defined as dedicated, shared, and spanned is shown in Figure 5-3.
Figure 5-3 IBM Z CSS: Channel subsystems with channel spanning
In the sample, the following definitions of a channel path are shown:
•CHPID FF, assigned to PCHID 20A, is dedicated access for partition 15 of LCSS0. The same applies to CHPID 00,01,02 of LCSS0, and CHPID 00,01,FF of LCSS1.
•CHPID 03, assigned to PCHID 20E, is shared access for partition 2, and 15 of LCSS0. The same applies to CHPID 05 of LCSS1.
•CHPID 06, assigned to PCHID 120 is spanned access for partition 1, 15 of LCSS0, and partition 16, 17 of LCSS1. The same applies to CHPID 04.
Channel spanning is supported for internal links (HiperSockets and IC links) and for certain types of external links. External links that are supported on z15 servers include FICON Express16SA, FICON Express16S+, FICON Express16S, FICON Express8S, OSA-Express7S, OSA-Express6S, OSA-Express5S, and Coupling Links.
The definition of LPAR name, MIF image ID, and LPAR ID are used to identify an LPAR by the channel subsystem to identify I/O functions from different LPARs of multiple LCSSs, which support the implementation of these dedicated, shared, and spanned paths.
An example of definition of these LPAR-related identifications is shown in Figure 5-4.
Figure 5-4 CSS, LPAR, and identifier example
LPAR name
The LPAR name is defined as partition name parameter in the RESOURCE statement of an I/O configuration. The LPAR name must be unique across the server.
MIF image ID
The MIF image ID is defined as a parameter for each LPAR in the RESOURCE statement of an I/O configuration. It ranges 1 - F, and must be unique within an LCSS. However, duplicates are allowed in different LCSSs.
If a MIF image ID is not defined, an arbitrary ID is assigned when the I/O configuration activated. The z15 server supports a maximum of six LCSSs, with a total of 85 LPARs that can be defined.
Each LCSS of a z15 server can support the following numbers of LPARs:
•LCSS0 to LCSS4 support 15 LPARs each, and the MIF image ID is 1 - F.
•LCSS5 supports 10 LPARs, and the MIF image IDs are 1 - A.
LPAR ID
The LPAR ID is defined by a user in an image activation profile for each LPAR. It is a 2-digit hexadecimal number 00 - 7F. The LPAR ID must be unique across the server. Although it is arbitrarily defined by the user, an LPAR ID often is the CSS ID concatenated to its MIF image ID, which makes the value more meaningful for the system administrator. For example, an LPAR with LPAR ID 1A defined in that manner means that the LPAR is defined in LCSS1, with the MIF image ID A.
5.2 I/O configuration management
The following tools are available to help maintain and optimize the I/O configuration:
•IBM Configurator for e-business (eConfig)
The eConfig tool is used by your IBM representative. It is used to create configurations or upgrades of a configuration, and maintains tracking to the installed features of those configurations. eConfig produces reports that help you understand the changes that are being made for a new system, or a system upgrade, and what the target configuration looks like.
•Hardware configuration definition (HCD)
HCD supplies an interactive dialog to generate the IODF, and later the IOCDS. Generally, use HCD or Hardware Configuration Manager (HCM) to generate the I/O configuration rather than writing I/O configuration program (IOCP) statements. The validation checking that HCD runs against a IODF source file helps minimize the risk of errors before an I/O configuration is activated.
HCD support for multiple channel subsystems is available with z/VM and z/OS. HCD provides the capability to make dynamic hardware and software I/O configuration changes.
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Note: Certain functions might require specific levels of an operating system, PTFs, or both.
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•Consult the appropriate fix categories:
– z15 T01: IBM.Device.Server.z15-8561
– z14 M0x: IBM.Device.Server.z14-3906
– z14 ZR1: IBM.Device.Server.z14ZR1-3907
– z13: IBM.Device.Server.z13-2964
– z13s: IBM.Device.Server.z13s-2965
•HCM
HCM is a priced optional feature that supplies a graphical interface of HCD. It is installed on a PC and allows you to manage the physical and logical aspects of a mainframe’s hardware configuration.
•CHPID Mapping Tool (CMT)
The CMT helps to map CHPIDs onto PCHIDs that are based on an IODF source file and the eConfig configuration file of a mainframe. It provides a CHPID to PCHID mapping with high availability for the targeted I/O configuration. It also features built-in mechanisms to generate a mapping according to customized I/O performance groups. More enhancements are implemented in CMT to support z15 servers.
The CMT is available for download from the IBM Resource Link website.
The configuration file for a new machine or upgrade is also available from IBM Resource Link. Ask your IBM technical sales representative for the name of the file to download.
5.3 Channel subsystem summary
z15 servers support the channel subsystem features of multiple LCSS, MSS, and the channel spanning that is described in this chapter. The channel subsystem capabilities of z15 servers are listed in Table 5-1.
Table 5-1 z15 CSS overview
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Maximum number of CSSs
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6
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Maximum number of LPARs per CSS
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CSS0 - CSS4: 15
CSS5: 10
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Maximum number of LPARs per system
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85
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Maximum number of subchannel sets per CSS
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4
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Maximum number of subchannels per CSS
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255.74 K
SS0: 65280
SS1 - SS3: 65535
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Maximum number of CHPIDs per CSS
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256
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