/COPY_SOURCE {ddcu: | DSAnnnn:} (Alpha/Integrity servers only)

    Specifies which source member of a shadow set will be used as
    the source for read data during full copy operations when a third
    member is added to the shadow set. This qualifier affects only
    copy operations that do not use disk copy data (DCD) commands.

    Some storage controllers, such as the HSG80, 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 lets you force all reads from a
    single, specified source member for a copy operation.

    If only a shadow set (DSAnnnn:) is specified, 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 shadow set member (ddcu:) is specified, 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.