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A virtual control file (VCF) is an area of virtual storage located in a pre-selected CA MIM address space. The internal structure of the VCF is exactly the same as the internal structure of a DASD control file. The CA MIM address space selected to manage the VCF is known as the master system. Eligible master systems are those that have connectivity to all systems participating in the MIMplex.
Client systems request the VCF through CTC devices (CTCONLY, CTCDASD) or through the z/OS Cross System Coupling Facility (XCF). Once the master system receives the request, it transports the VCF back to the requesting client system. The master system is charged with managing the VCF. Only the master system knows which system has the VCF at any given moment. Once the client system receives the VCF, it is updated and transported back to the master. The master system then sends the VCF to the next waiting requestor (client system).
VCF access requests are issued “on demand” by each CA MIM address space. On demand means when an enqueue request needs to be propagated globally, CA MIM immediately demands, or requests, the VCF. The MODE parameter setting determines when these requests are processed. The master system services these VCF access requests on a first in, first out (FIFO) basis. This philosophy ensures that the systems with the most enqueue activity receive the best possible service.
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