Elements of System z I/O

Channel Subsystem (CSS) is where I/O operations takes place. It communicates between internal-external channel, Control Units (CU's) & devices.

It's main elements are briefly discussed below:

Channel Path - It is the interface between the processor(aka host) and one or more CU's at the storage device end. Any MVS command or data passes through these channel paths to process a I/O request. Max. allowed channel paths - 256

Subchannels - A logical representation of a device to the program and contains the information that is required to sustain a single I/O operation. Every defined device address to a host has a unique Subchannel ID (SSID).

There's a maximum of 3 Subchannel sets (xx00-yyFF) on a z16 A02.

Channel Path Identifier - Each channel path (discussed above) is assigned a channel path identifier (CHPID) value that uniquely identifies that path. On system z, a CHPID is assigned to a physical slot (or port) either through HCD or IODF.

Control Unit - CU provides logical capabilities to operate & control a I/O device. CU can be present physically or logically integrated with the Storage device, the CSS or in the host system itself.

Multiple CSS - To scale up I/O capability, multiple CSS can be accommodated. This allows more number of supported LPAR's, channels and devices available to the system.

Upto 6 Logical CSS's, each with max. 4 SS's and up to 256 channels are supported.

Physical Channel ID - A PCHID denotes the physical location of a channel-type interface. It's number is based on Peripheral Component Interconnect Express+ (PCIe+) I/O drawer, Peripheral Component Interconnect Express (PCIe) I/O drawer, or I/O drawer location; the channel feature slot number; and the port number of the channel feature. In IODF definition, CHPID's are mapped under PCHID.

PCHID Dummy Report for IBM z16 A02
---------------------------------------------------------------
Logical Partition ID: LP01
CHPID  PCHID   Adapter ID  Type      Description
------ ------  ----------  -------   ------------------------
00     050     A01         FICON     Fibre Channel Port
01     051     A02         FICON     Fibre Channel Port
02     0A0     A03         OSA       Open Systems Adapter
03     0A1     A04         OSA       Open Systems Adapter

Logical Partition ID: LP02
CHPID  PCHID   Adapter ID  Type      Description
------ ------  ----------  -------   ------------------------
04     0B0     B01         ROCE      RDMA Over Converged Ethernet
05     0B1     B02         ROCE      RDMA Over Converged Ethernet
06     0C0     C01         CRYPTO    Crypto Express Adapter
07     0C1     C02         CRYPTO    Crypto Express Adapter
        

Use-case:

In this example, there are two CSS (0 & 1) each has three LPAR's with respective Multiple Image Facility (MIF) identifier. Each pair of CHPID's are shared between two LPAR's. Each CHPID is assigned to a designated PCHID.

Mapping between CHPID's & PCHID's are done through IBM's CHPID mapping tool; whose output serves as input to the IODF.