D_UNITS
off = (lbn-BASE_DINDIR) / D_UNITS;
off = (lbn/D_UNITS) - (up/D_UNITS);
up = -(((lbn-ULFS_NDADDR) / D_UNITS) * D_UNITS + ULFS_NDADDR);
off = (lbn-ULFS_NDADDR) % D_UNITS;