This field is incremented by one each time a modified buffer is
    written back to disk. Its value should be equal to the sum of
    the 14 following fields. These fields as well as the SPAM page
    field provide further detail about buffer unmark activity. Write
    operations of buffers that contain SPAM pages are included in the
    total although they are also counted separately by the SPAM page
    field.

    Note that unmarking one buffer may entail more than one I/O.