/sys$common/syshlp/helplib.hlb SET, DEVICE, Qualifiers, /COPY_SOURCE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /COPY_SOURCE (ddcu:,DSAnnnn:)

    Controls whether one or both source members of a shadow set are
    used as the source for read data during full copy operations,
    when a third member is added to the shadow set. This only affects
    copy operations that do not use DCD operations.

    HSG80 controllers have a read-ahead cache, which significantly
    improves single-disk read performance. Copy operations normally
    alternate reads between the two source members, which effectively
    nullifies the benefits of the read-ahead cache.

    This qualifier allows you to force all reads from a single source
    member for a copy operation.

    If the shadow set is specified, then all reads for full copy
    operations will be performed from the disk that is the current
    "master" member, regardless of physical location of the disk.

    If a member of the shadow set is specified, then that member will
    be used as the source of all copy operations. This allows you to
    choose a local source member, rather than a remote master member.