/COPY_SOURCE ddcu:
       /COPY_SOURCE DSAn:
       /COPY_SOURCE /ALL

    Note: Do not include any HBMM-specific qualifiers in a command
    with this qualifier; doing so causes the command to fail.

    Specifies which source member of a shadow set to use as the
    source for read data during full copy operations when a third
    member is added to a shadow set that contains two full members.
    This qualifier affects only those copy operations that do not
    use disk copy data (DCD) commands. The source specified by this
    qualifier persists until the shadow set is dismounted.

    Some storage controllers, such as the HSG80, have a read-ahead
    cache, which significantly improves a device's 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 the duration of a copy
    operation.

    In addition to improving copy performance, /COPY_SOURCE can
    be used to prevent read operations from a specific shadow set
    member that is considered unreliable. By specifying only the
    healthy shadow set member, the copy operations can continue to
    completion. The unreliable shadow set member can be removed once
    the copy operation completes successfully.

    If a shadow set (DSAn:) is specified, all reads for full copy
    operations are performed from the device that is the current
    "master" member, regardless of the physical location of that
    device.

    If a shadow set member (ddcu:) is specified, that member is used
    as the read source for all copy operations. This setting allows
    you to choose any source member. For example, you can choose a
    source member that is at the same site as the member being added,
    rather than using a master member that is not at the same site.

    If /ALL is specified, all reads for full copy operations on all
    currently mounted virtual units are performed from the master
    member.