VMS Help SET, DEVICE, Qualifiers, /READ_COST  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /READ_COST=n ddcu:

    Allows you to modify the default "cost" assigned to each member
    of a shadow set, so that reads are biased or prioritized toward
    one member versus another.

    The valid range for n is 1 to 65,535 units.

    The device specified must be a member of a shadow set that is
    mounted on the node where the command is issued.

    The shadowing driver assigns default READ_COST values to shadow
    set members when each member is initially mounted. The default
    value depends on the device type, and its configuration relative
    to the system mounting it. There are default values for a
    DECRAM device; a directly connected device in the same physical
    location; a directly connected device in a remote location;
    a DECram served device; and a default value for other served
    devices.

    The value supplied by this qualifier overrides the default
    assignment. The shadowing driver adds the value of the current
    queue depth of the shadow set member to the READ_COST value and
    then reads from the member with the lowest value.

    Different systems in the cluster can assign different costs to
    each shadow set member.

    If the /SITE command qualifier has been specified, the shadowing
    driver will take site values into account when it assigns default
    READ_COST values. Note that in order for the shadowing software
    to determine whether a device is in the category of "directly
    connected device in a remote location," the /SITE command
    qualifier must have been applied to both the shadow set and the
    individual device.

    Reads requested for a shadow set from a system at site 1 are
    performed from a shadow set member that is also at site 1. Reads
    requested for the same shadow set from site 2 can read from the
    member located at site 2.