Layout per volume
Up to now the layout was defined for the exportd daemon and so did apply
to every volume handle by this daemon. Today a layou is still defined
at daemon level but this is the defauklt layout that will be applied to
its volumes. It is possible to over ride this value by defining a layout
on a volume.
In the following example the default layout is 2, but volume 1 has
a layout of 0.

layout = 2 ;
volumes =
      (
        {
            vid = 1;
            layout = 0;
            cids=
            (