Channel configuration considerations

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The following configuration rules determine how logical paths are established for ESCON and FICON® channels.
  • A channel initially attempts to establish logical paths:
    • For channels configured online to logical partitions that are activated at POR. Shared channels attempt to establish logical paths only for those activated logical partitions with the channel configured online.
    • When a logical partition is activated.
    • When configured online (if previously configured offline).
  • A channel that cannot initially establish a logical path can reattempt to establish a logical path if the channel detects or is notified of:
    • A change in the state of a control unit
    • A change in the state of a link or port
    • A dynamic I/O configuration change that frees previously allocated logical paths.
  • A channel cannot establish a logical path or its logical path is removed if:
    • A logical partition is deactivated. A shared channel continues to operate for any other remaining activated logical partitions to which it is defined. Logical paths to those logical partitions remain established.
    • The director that connects the channel to the control unit blocks either the channel port or control unit port used in the path.
    • The director that connects the channel to the control unit prohibits the dynamic connection or communication between the channel port and the control unit port used in the path.
    • A link involved in the path fails or is disconnected. When a shared channel is affected by any of the following circumstances, each logical partition sharing the channel is equally affected:
      • The blocking of a port
      • A prohibited dynamic connection or communication
      • A failing or disconnected link
      All logical paths using the port or link are removed (regardless of the logical partition with which they are associated).
    • The channel is configured offline. When a shared channel is configured offline for a logical partition, it continues to operate for any other logical partition that has the channel configured online. Logical paths to these other logical partitions remain established.
    • Power to the channel, control units, or directors in the configuration is turned off.
  • A shared channel cannot establish a logical path to a control unit for a logical partition that cannot access any of the I/O devices on the control unit. The PART, PARTITION, or NOTPART keyword on the IODEVICE statement specifies which logical partitions are permitted to access a device.
  • A channel does not establish a logical path to a control unit if no devices are defined on the control unit.