How HCD arranges devices into groups in an IODF

IODF versions up to V4 contained a separate device definition record for each single device. To reduce the size of IODFs and to improve the processing performance of large configurations, z/OS V1.7 HCD uses a new IODF format V5, arranging single devices into a device group, if they have the following characteristics in common:

• The device numbers of all devices are in consecutive sequence.
• All devices of the group have the same device type (unit, model) and attribute values (Serial-#, Description, VOLSER).
• All devices of the group are attached to the same control unit(s).
• All devices of the group are connected to the same processors/channel subsystems and have the same corresponding processor-specific attributes.
• All devices of the group are connected to the same operating systems and have the same corresponding OS-specific attributes (device type, parameters, features, console definition, subchannel set number).
• For each OS and each eligible device table (EDT) in the OS, all devices of the group are connected to the same esoterics.

Devices that adhere to the these rules are aggregated into device groups containing the maximum number of applicable devices. If you apply a change on one or more devices from a group, HCD checks how to rearrange the devices and device groups contained in the IODF in order to achieve the best possible organisation of devices into groups again according to the specified rules. Another example may be that you change the attributes of a group in a way that two previously similar groups now must be merged into one group. However, the required rearrangements are not necessarily performed by HCD at once, but may be delayed to an appropriate point in time.