SPAN_ROW_SIZE
SPAN_ROW_SIZE(drv_map, ld, span) : raid->rowSize;
arm = mega_mod64(stripe, SPAN_ROW_SIZE(map, ld, span));
rowMod = mega_mod64(row, SPAN_ROW_SIZE(map, ld, span));
armQ = SPAN_ROW_SIZE(map, ld, span) - 1 - rowMod;
if (arm >= SPAN_ROW_SIZE(map, ld, span))
arm -= SPAN_ROW_SIZE(map, ld, span);