System-managed duplexing rebuild is a process managed by the system that allows a structure to be maintained as a duplexed pair. The process is controlled by CFRM policy definition as well as by the subsystem or exploiter owning the structure. The process can be initiated by operator command (SETXCF), programming interface (IXLREBLD), or be MVS-initiated. Note that user-managed duplexing rebuild is controlled and initiated in the same manner as system-managed duplexing rebuild but is managed by the subsystem or exploiter owning the structure and applies only to cache structures.