/BLOCK_SIZE=n

    Specifies the output block size, in bytes, for data records in
    BACKUP save sets and in disk-to-disk copies. You can specify a
    block size between 2048 and 65,535 bytes. BACKUP may adjust this
    value according to the constraints of the BACKUP format.

    The default block size for magnetic tapes is 8192 bytes. The
    default for disks is 32,256 bytes.

    For a disk-to-disk operation, the block size specifies the
    internal size of the copy buffers. The default block size in
    this case is 33,040 bytes.