VMS Help V73 Features, Programming Features, Multipath System Services  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The new Multipath system services provide the capability to
    return path information and allow you to enable, disable, and
    switch specific I/O paths to any device.

    The concept of multiple I/O paths to storage devices was
    introduced in OpenVMS Version 7.2-1. It is now possible to select
    more than one I/O path to a device in the event that the path in
    use should fail.

    To assist in decision making when configuring a system's I/O
    structure, the following DCL commands were made available to
    allow you to display I/O path information and change the current
    settings affecting these paths:

    o  SET DEVICE device-name/PATH=path-description-string/SWITCH

    o  SET DEVICE device-name/PATH=path-description-string/[NO]ENABLE

    o  SHOW DEVICE/MULTIPATH device-name

    In OpenVMS Version 7.3, the capability to return path information
    and allow you to enable, disable, and switch specific I/O paths
    to any device is now implemented in the following new system
    services:

    o  SYS$DEVICE_PATH_SCAN

       This service returns path information for a given Multipath
       I/O device. Each call to the service returns the name of one
       of the paths to the device. A context argument is used to
       maintain continuity between calls. This mechanism is similar
       to the one currently used for SYS$GETDVI.

    o  SYS$SET_DEVICE[W]

       Use this service to switch the selected path that handles I/O
       to a device, or to enable or disable a path for future use in
       the event of failover. When switching a path, the path change
       is initiated at the time the request is made by the system
       service.

       The current functions of this service include forcing an
       immediate path switch and enabling or disabling paths.

       A synchronous version of this service, SYS$SET_DEVICEW, is
       also provided. This service returns to the caller only after
       the path switch attempt has been made. Should the path switch
       fail, an error condition is returned to the caller.

       Currently, $SET_DEVICE allows only one valid item list entry.

    For additional information, refer to the OpenVMS System Services
    Reference Manual.