The SYSMAN IO REBUILD command rebuilds the system's
    device configuration tables by reading and parsing the
    SYS$SYSTEM:SYS$USER_CONFIG.DAT and SYS$SYSTEM:SYS$CONFIG.DAT
    files.

    To debug modifications to the SYS$SYSTEM:SYS$USER_CONFIG.DAT
    file, you can use the SYSMAN IO REBUILD and SYSMAN IO
    AUTOCONFIGURE commands to load drivers without having to reboot.
    Once you load a driver for an adapter, however, you cannot reload
    it without rebooting the system.