This topic discusses utility devices that XRC uses to read from a primary storage control. When an SDM reader task reads changed data from a primary subsystem, it must use a device address available on that subsystem. There is an SDM reader subtask for each storage control session and, at a minimum, there is one storage control session associated with each SSID. The SDM requires a device address in each storage control session, which is called a utility device or reader.
Any device address that is associated with the storage control session can be used as the utility device. If the device is used for any significant amount of primary system I/O activity, application activity will block the SDM from transferring data. This situation can result in the SDM experiencing performance delays.
Conversely, when the microcode designates a particular device as its utility device, it has a special status that prevents if from being used by the primary systems. This may also have an impact on primary systems.
You can define utility devices by using the XADDPAIR command for a single cylinder volume, with the XRCUTL specified for the secondary volume. If custom volume sizes are supported by the subsystem, then single cylinder volumes can be created for use as utility devices. You can define multiple utility volumes for a storage control session. (Eighty percent or more of customers use fixed utility devices.)