A virtual control file (VCF) is an area of virtual storage that is located in a pre-selected CA MIM address space. The CA MIM address space that is selected to manage the VCF is known as the master system. A system is master eligible if it has connectivity to all systems participating in the MIMplex.
External client systems access the virtual control file through CTC devices that are connected to the VCF master system. The master system itself is a client system, and accesses the virtual control file through memory operations, without any external input/output operation.
Once the master system receives the request, it transports the VCF back to the requesting client system. 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). The master system services VCF access requests on a first in, first out (FIFO) basis. This philosophy ensures that the systems with the most global activity receive the best possible service.
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