Shared CTC unit address ranges

Edit online
IOCP calculates the number of unit address ranges for a shared CTC channel path by performing the following processing for each physical control unit attaching to the channel path:
  • If a physical control unit has no devices, one unit address range is counted.
  • If a physical control unit has devices, IOCP processes the candidate list for each device. (The candidate list is determined by the PART, PARTITION, or NOTPART keyword in the IODEVICE statement.) A device candidate list determines which logical partitions (LPs) can access the device. It can be independent of the candidate list for the CTC channel path assigned to the device. IOCP determines the number of unique LPs contained in the device candidate lists. The number of unique LPs is the number of unit address ranges for the given physical control unit. The maximum number of unit address ranges for a shared ESCON CTC channel path is 120.

    If you do not specify the PART, PARTITION, or NOTPART keyword in an IODEVICE statement that is assigned to a shared CTC channel path, the device candidate list defaults to the LPs in the candidate list of the CTC channel path. Because the maximum number of physical control units and unit address ranges for a shared CTC channel path are both 120, you cannot define 120 physical control units on a shared CTC channel path if one of its control units has more than one unit address range.

    To control the number of unit address ranges in a physical control unit assigned to a shared CTC channel path, perform one or both of the following actions:
    • Specify a CHPID candidate list that contains only the LPs that access the shared CTC channel path. If the CHPID candidate list does not contain all the LPs in the configuration, devices that do not specify a device candidate list have fewer unit address ranges for their physical control unit.

      For example, if your configuration has 10 LPs and a CTC channel path is shared by 5 LPs in its candidate list, each device assigned to the shared CTC channel path has a device candidate list that defaults to 5 LPs. The physical control unit assigned to the shared CTC channel path has 5 unit address ranges. Therefore, the maximum number of physical control units that you can define on the shared CTC channel path is only 24 instead of 120 because this configuration first reaches the maximum of 120 unit address ranges on the shared CTC channel path.

    • Specify a device candidate list for each device assigned to a physical control unit. If the device candidate lists of all the devices assigned to a physical control unit do not contain all the LPs in the configuration, fewer unit address ranges are defined for the physical control unit.

      For example, if your configuration has 10 LPs and all of them share a CTC channel path, each device assigned to the shared CTC channel path has a device candidate list that defaults to all 10 LPs. If you specify device candidate lists so that only 4 unique LPs can access any of the devices on a physical control unit, the control unit has 4 unit address ranges. Therefore, the maximum number of physical control units that you can define on the shared CTC channel path is only 30 instead of 120 because this configuration first reaches the maximum of 120 unit address ranges on the shared CTC channel path.

    You can increase the number of physical control units you can define on a shared CTC channel path up to the maximum of 120 by decreasing to one the number of unit address ranges on the control units assigned to the CTC channel path.