The number of Virtual Control Units can be between 1 and 8. The maximum number of Virtual Devices that can be assigned to a Virtual Control Unit is 100.
We recommend that you define two or more Virtual Control Units. This ensures that you can restart portions of the subsystem, effectively guarding against subsystem outages to address a Virtual Device or Virtual Control Unit problem. Multiple Virtual Control Units also reduce enqueue contention. In particular, the SYSZTIOT enqueue issued by dynamic allocation is spread over more than one address space, which reduces the time spent on virtual mounts and dismounts.
CA Vtape manages the startup and shutdown of Virtual Control Unit subaddress spaces, and diagnostic information is gathered for all Virtual Control Unit address spaces when dumps are generated internally or by the SVTn DUMP console command.
The VirtualControlUnit attribute in the VTDRIVE member of parmlib defines the number of Virtual Control Units for a subsystem. The Online and Offline (or both) attributes coded immediately following the VirtualControlUnit attribute indicate which Virtual Devices belong to that Control Unit. The following VTDRIVE example defines four Virtual Control Units each controlling four Virtual Devices:
<VirtualDeviceList> VirtualControlUnit=1 Online=0570-0573 VirtualControlUnit=2 Online=0574-0577 VirtualControlUnit=3 Online=0578-057B VirtualControlUnit=4 Online=057C-057F
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